Heat transfer enhancement has been widely used in recent years. Heat exchangers with enhanced heat transfer work at a lower velocity or reduced driving force temperature and/or smaller size. Many ways exist to achieve these goals. One of them is utilization of inserts. Both heat transfer and pressure drop increase by using inserts. There are many types of inserts, like twisted type, wire coil, wire mesh, etc. Some inserts are not beneficial due to their relatively high pressure drop. In this study, the effect of flexible filaments on enhancement of heat transfer and pressure drop is investigated. Also a Performance Evaluation Criterion has been defined and applied to evaluate the economic benefit of these filaments. In this project, the effect of filaments on heat transfer and pressure drop in a tube of uniform wall temperature is investigated. Steam is used as a heat source to obtain a uniform wall temperature and the working fluid in the tube is water. The results are presented in two regimes of flow, i.e. laminar and turbulent. The range of Reynolds number for laminar flow is from 600 to 2300, and for turbulent flow is from 3000 to 10000. Three kinds of filaments are used in this project: Polypropylen, Polyester, and Cotton. Density of these filaments when dry is 0.91, 1.39, and 1.55 respectively. Filaments were installed in four array types in the pipe. Furthermore, experiments were carried out using three lengths of filaments. The results of our experimental tests have shown that use of these flexible filaments leads to enhanced heat transfer and pressure drop, whose extents depend on the type of filament, array type and length of filament in the tube. It was found that filaments with higher density have greater effect on heat transfer and pressure drop. Also, filaments of shorter length are more effective. Furthermore, the more the number of filaments in an array, the more effective on heat transfer and pressure drop is. In addition heat transfer enhancement increases with increase in the diameter of filament. Among the configurations investigated, the array consisting of three cotton filament of 30 centimeters length and distributed uniformly in the tangential direction has performed the best. 51% increase in the Nusselt number was observed when this arrangement was used in laminar flow. Furthermore, in turbulent flow, the Nusselt number increases by 46%. The Performance Evaluation Criterion (P.E.C) of 0.61 was achieved. Keywords: Internal flow, Flexible filaments, enhancement of heat transfer, Inserts.