This study presents a method for the fabrication of ultrafine polymeric nano?bers utilizing centrifugal and electrostatic forces simultaneously. To reduce the diameter and variability of polyacrylonitrile (PAN) and poly-L-lactic acid (PLLA) nanofibers, a unique electro-centrifuge spinning device was designed using rotating nozzle and collector, whereas the fabrication process (spinning head) was skillfully sealed from ambient airflow. The polymer solution was continuously delivered by a rotating nozzle and the nanofibers were collected by a rotating cylindrical collector at the same rotational speed as the nozzle. Field emission scanning electron microscope (FESEM) results demonstrated that this method has a significant effect on the quality and fineness of nanofibers. The diameters of nano?bers were controlled by adjusting the rotation speed of spinning head. The effect of the rotation speed on the morphology of the nanofibers fabricated by this device was also evaluated. In order to provide a useful context for the current nanofiber production method, nanofibers obtained in this method were compared with those produced by other methods. The results show that air-sealed-centrifuge-electrospinning system (ASCES) is a facile method for the fabrication nano?bers with smaller diameters and high uniform structures. The Taguchi method was adopted to optimize the fabrication condition of solid state polymerized polyamide ( PA66) nanofibers with respect to minimize nanofiber diameter. Air-sealed centrifuge electrospinning (ASCES) was used for preparing high quality nanofibers. In this study, solution concentration, rotational speed, syringe content, and applied voltage were selected as key parameters a?ecting the diameter of the fabricated nanofibers. The morphology of the electrospun nanofibers was analyzed by using the field emission scanning electron microscope (FESEM). Nanofiber diameters ranging from 20 to 250 nm were successfully controlled by the Taguchi method, and the statistical data based on experimental results indicated that the syringe content and rotational speed had greater in?uences (35.76% and 23.93%, respectively) on nanofiber diameter than did solution concentration and voltage (21.36% and 11.49%, respectively). Lower syringe content as well as higher rotational speed decreased the nanofiber diameter. The minimum concentration decreased the diameter of PA66 nanofibers. Also, applied voltage had no significant effect on nanofiber diameter. Centrifuge spinning suffered from a strong air resistance which leads to a more deflected jet as well as its rapidly solvent evaporation resulting in thicker nanofibers. A comparison was drawn between the trajectory of Newtonian jets fabricated by centrifuge spinning and air-sealed centrifuge spinning. The captured images of the jet trajectory using a high speed camera showed that non-isolated jets were more curved than isolated jets due to air resistance. A non-linear analysis of the Navier-Stokes equations was carried out and one-dimensional equations were derived using asymptotic techniques. The experimental results were compared to numerical solutions of equations. There was fairly good agreement between the isolated jet trajectory and the model-predicted one, but there were differences between the non-isolated jet trajectory and the simulation results. The non-isolated jet curved more compared to the others due to air drag. Simulation results showed that decreasing the Roy number which can be translated to an increase in angular velocity, reduced the overall size of the fiber radius, contracted the trajectory, and increased the tangential velocity and thus the production rates. The centrifugal force was increased significantly by increasing the speed of rotation. Thus, the higher the centrifugal force, the greater the extension of polymeric jet, which results in thinner ?ber diameters. The simulation provided an insight into the behavior expected during fiber formation with the inclusion of viscosity. The influence of the Reynolds number showed that decreasing the Reynolds number which related to an increase in viscosity, increased the fiber radius, contracted the trajectory, and reduced the tangential velocity and thus production rates. Actually higher viscosity makes the extension of the jet more difficult and results in the formation of a thicker fiber. Also, it was found that there is no difference between radius, trajectory, and tangential velocity of fibers fabricated at various Weber numbers. Simulation results showed that there is no effect of applied voltage on radius, trajectory, and tangential velocity of fibers. Keywords: Nanofibers, Morphology, Air-sealed-centrifuge-electrospinning system, Taguchi method, Curved liquid jet, Navier-Stokes equations, Non-linear analysis, Asymptotic expansions