The objective of this study is to find the optimal size of splined shaft in IMS module. Two methods were used in this study. One is for the investigation of effect of indentation process on the tubular shaft yoke, and another is for the investigation of effect of indentation process on splined shaft. The spline outer size of splined shaft was increased from 0.00mm to 0.20mm. The simulation results were analyzed and compared in terms of under-fill, metal flow, effective strain, Von-mises stress and load characteristics. The indentation load was increased with increasing of spline size. However, in case of 0.15mm outer diameter increasing, the separation load was decreased. The case of 0.10mm diameter increasing was the best spline size based on the low indentation load and high separation load.