For modern people who sit for a long time in daily-lives, back pain is a critical disease. The back pain is closely related to bad posture, but there are limited systems to monitor sitting posture. Moreover, those systems have some shortcomings to be used in daily-lives. Wearing inertial measurement unit bothers the user’s natural movement and motion camera needs wide-open space. In this study, we proposed a method to estimate sitting posture based on ground reaction force that could be measured without wearing sensors or the need of large space. We modeled the upper-body as a multi-segment rigid body and developed several assumptions, and ten subjects participated in the sitting test with various postures to verify our model. The results showed sitting trunk angle could be reasonably estimated based on GRF under straight posture condition. It implies that the proposed method could be functionally suited as a posture monitoring system.