Abstract: A thorough investigation of the relationship between joint line and knee biomechanics after total knee arthroplasty (TKA) will guide the TKA surgery, and will improve the restoration of knee function. Different joint line models of two TKA surgeries (with patellar resurfacing and without patellar resurfacing) were developed and simulated in commercial finite element software. The simulation results show that if the joint line is too high or too low, the knee biomechanics will be affected, resulting in mal-motion of patella and high prosthesis stress. We draw the conclusion that the joint line cannot be moved down and the max displacement of moving up cannot be over 4mm. The simulation results provide basic method and data for assessing TKA surgery and prosthesis optimal design.