Absract Chopper’s tail power transmission consists of several hollow shafts which are connected by flexible coupling. Input power of this system is provided by main gearbox and in the end of the tail power is transmitted to 42 degree gearbox. The length of the system is 6.5 meter and works in the working speed 4200 rpm. This system consists of four shafts which three of them have the same length. These four shafts are connected together via three flexible couplings. These couplings are flexible both in lateral and axial directions. System’s modeling needs shaft and flexible coupling modeling in software. Split shaft’s modeling in software is not feasible because this type of flexible coupling consists of approximately 20 pieces and its modeling is complex and unreliable. For solving this problem, flexible coupling’s model should be replaced by another finite element model. After analyzing different model, flexible coupling’s model will be created using replaced spring-damper model.After carrying out experimental model test under whole system of split drive shafts with flexible coupling, its dynamical properties will be determined, including natural frequencies, mode shapes and damping coefficient. Using experimental model test’s result used model will be improved. This improvement is carried out with replaced model coefficient so after performing system’s modal analysis in software, experimental modal analysis results is compatible with modal analysis results performed in software. Eventually all software modal analysis results were compatible with experimental modal test results except one longitudinal direction in experimental test. With considering the results, system’s software model is verified. For modeling system in software, the boundary conditions governing the system during the test were applied. Using finite element theory mathematical model of system is extracted and using this model, modal analysis of system can be done theoretically. For optimization the structure of split drive shafts with flexible couplings, some of system’s properties will be considered as design parameters and considering working space of system on chopper constraints of optimization will be determining. Also objective function of optimization will be determined in such a way that natural frequencies of system are kept away from stimulating frequencies. stimulating frequencies can be due to factors like working rotational speed, frequency created by bearing etc. Using Genetic Algorithm optimization was done and first natural frequency was increased and the other natural frequencies grew up and were kept away from stimulating frequencies due to working rotation speed of the system and also bearings.