In this paper, steady RANS (Reynolds-Averaged Navier-Stokes) numerical modeling of the return system of a centrifugal compressor is carried out to investigate the effects of various geometrical parameters on the aerodynamic performance of the return system. The effects of using splitter vanes in the return-channel, the width ratio of the return channel and the diverging angle of the walls are examined. Numerical results revealed that using splitter vanes can enhance the stage averaged pressure ratio and polytropic efficiency by 0.74 and 3.21%, respectively. Results showed that increasing the width ratio can enhance the efficiency and pressure ratio near the choke region, however, it has marginal effects on the performance at low flow coefficients. Results also showed that, due to the negligible effects of the diverging angle on the stage performance, using diverging hub wall can reduce the total axial length of the multistage compressor. Finally, a new return system is designed based on the combination of the best geometrical parameters found in the current study. Simulation results demonstrated that the average pressure ratio and efficiency of the compressor have been improved by 0.99% and 4%, respectively, by the newly developed return system compared to the original design.