In this study, the effect of the mechanical parts consisting of Tachometer/Gearbox, Digital Encoder, Input and Output Potentiometers, a brake disc, transmission belt, and some couplings which are directly connected to the shaft of a DC servo motor is investigated. The appropriate model of the DC servo motor can be achieved by modeling the effect of the mechanical parts correctly. The contribution of these mechanical parts is added to the mathematical model of the DC servo motor as new parameters. These parameters are estimated by using the Particle Swarm Optimization (PSO) algorithm since the convergence speed of the PSO is high. The result of the PSO algorithm is confirmed with experimental results. According to the experimental results, the obtained mathematical model including the effect of mechanical parts represents the real system more accurate.