In this master thesis, a mathematical model of a hydraulic servo system for a manipulator robot is completed by using several different methodologies. The models proposed are particularly tuned for the DeLaval VMS robotic arm. The parameter identification of the robotic arm is accomplished by dividing the model into several subsystems and investigating each system separately by using catalogue data, experimental data and construction drawings. Furthermore, the assumptions are proposed based on the literature review and the expertise of in-house engineers. After completion of parameter identification several different mathematical models including linear and nonlinear methodologies are introduced. It is demonstrated that the improvednonlinear model can successfully mimic the movement of the robotic arm with relatively small errors and it is found to be fairly reliable. Moreover, the errors incurred when chamber pressures are compared with experimental data are found to be relatively small. Furthermore, the improved linear model have successfully delivered an accurate position estimation especially for the medium valve opening, while the chamber pressures are relatively less accurately predicted. The study further carries out sensitivity (uncertainty) analyses to investigate the crucial parameters of the model since it is sometimes very problematic to precisely estimate these parameters. It is found out that the flow coefficient and supply pressures have remarkable impact on the results of the simulations. Therefore, it is strongly advised that these parameters should be very carefully evaluated during the modeling process. Finally the bulk modulus models are compared and the influence of the bulk modulus is revealed.