PURPOSE: To evaluate the effect of the decrease in posterior slope of the tibia on the patellar height after TKA. MATERIALS AND METHODS: One hundred and fifty three cases of TKA (CR type: 43, PS type: 110) were evaluated to measure the posterior slope of the tibia and the patellarheight on lateral radiographs of the knee before and after surgery. Then, we analyzed a correlation between two measurements. RESULTS: The decrease in tibial posterior slope and patellar height was average 6.6 degrees and 1.4mm, there was a weak correlation (r= -0.293, p=0.000), as more decrease in tibial slope, less decrease in patellar height. Comparing CR group with PS group, the decrease of posterior slope of tibia and patella height was average 3.5 vs 7.8 degrees (p=0.000) and 2.2 and 1.2 mm (p=0.146) respectively. There were also correlations (r=-0.438, p=0.003 vs r=-0.228, p=0.017) between two decreases. CONCLUSION: It is considered that the decrease in tibial posterior slope is one of elements influencing in patellar height, and through preoperative radiological examination, estimation of expected decrease in tibial posterior slope is helpful in preserving patellar height in TKA.