Abstract: In this paper the method of determining upper magnitude limit following modified Gutenberg-Richter model is derived. Based on two commonly used models the modified magnitude-frequency relations and upper magnitude limits for various regions of Chinese mainland are determined. The result shows that there are nonlinear magnitude-frequency relations for earthquakes in Chinese mainland and the traditional normal G-R model can not reflect nonlinear feature of actual magnitude-frequency relations. While MGR model is suitable for expressing both linear and nonlinear magnitude-frequency relations, and is more suitable for dealing with moderate-strong earthquakes in various regions of Chinese mainland. When the magnitude-frequency relation is nonlinear, the upper magnitude limit determined by using MGR model is more reasonable.