Characterizing the relative performance between mobile bearing and fixed bearing knee prosthesis remains seen as a difficult task as the previous short-term and mid-term clinical studies disable to observe any evidence of superiority of one design over another. The aim of the present study is to characterize the mechanics comparison between both designs of prosthesis during deep flexional motion with tibial rotation. Three dimensional (3D) FE model of clinically used mobile bearing posterior stabilized (PS) prosthesis was developed from its CAD data. Explicit finite element model was used to simulate the dynamic loaded deep flexional motion from 0 to 135° with neutral and 10° tibial rotation. Fixed bearing prosthesis was represented by fixing the tibial insert to the tibial component. The fixed bearing design was found relatively sensitive to flexion motion and tibial rotation in terms of contact area and maximum shear stress as compared to the mobile bearing design. Tibial rotation increased the peak value of maximum shear stress up to 58 MPa for the fixed bearing, on the contrary, the mobile bearing maintained the peak value of maximum shear stress at 31 MPa even with tibial axial rotation. The influence of post-cam design was also discussed in this study. The mobile bearing has an ability to maintain conformity and relatively low shear stress during very deep flexion with tibial axial rotation in comparison to the fixed bearing.