Background: Floor sit-to-stand (FSTS) places a higher load on the knees than chair sit-to-stand (CSTS). It is difficult to experimentally measure the maximum knee joint force during sit-to-stand motion. Objectives: This study's objectives were twofold: firstly, to quantify the differences in knee joint force between FSTS and CSTS, and secondly, to identify the angles at which the maximum knee joint force occurs during these motions. Design: Computer simulation study. Method: This study was conducted on 4 adult male subjects in their 20s. The FSTS and CSTS motion trajectories of the subjects were acquired using 3- dimensional motion analysis equipment. Using these, the human body mass model of the program was modified according to the subject, and the knee joint force was calculated. Also, the knee angle at which the maximum knee joint force occurs was found. Results: When the subjects performed the FSTS motion, a knee joint force that was up to about 160% higher than that of the CSTS motion occurred, and the angle at which the maximum knee joint force occurred was different between the sitting sequence(FSTS motion: 56~58 degree, CSTS motion: 78~82 degree) and the standing sequence(FSTS motion: 98~100 degree, CSTS motion: 70~74 degree). Conclusion: By comparing FSTS motion with CSTS motion, it is expected that it can be used as a quantitative guide for the effect of motion similar to FSTS motion on the knee when prescribing exercise for the elderly or patients with knee-related lesions.