Abstract: Geometric factor calculations play an important role in the absolute calibration of radioactivity measurement systems and in the determination of the activity of radioactive sources. The Monte Carlo method can always calculate the geometric factors when there is no analytical algorithm available for geometric factor calculations. Few papers have reported the calculations of geometric factors when source and detector are both disc, but are un-coaxial and un-parallel. The program in this study is based on the Monte Carlo method and is written in the C++ programming language, and it can be used to calculate the geometric factor of a disc detector for a disc source. The detector and the source can be any size at any cases. This program integrated a variance reduction method, by comparing. The results with that calculated using MCNP5. It can be concluded that the program calculates geometric factors accurately, fast and conveniently. The geometric factor calculation program verified the accuracy of the pose of the detector which was applied in the NDP energy spectrum measurement system; furthermore, it corrected the detector position roughly.