To measure the coordinate of Center of Rotation(COR) for projection in an X-ray CT imaging system,calibrate the projection geometry coordinate system,and to eliminate the CT image artifacts caused by the positioning error of COR, this paper derives two lemmas based on the advantages and disadvantages of existing methods,i.e. ‘A particle is scanned a circle by a CT scanner, then The integral of the particle's projection addresses under each projection view equals to zero’ and ‘The sum of the crossing point coordinates of projection sine curves of random two particles equals to zero’. According to the above lemmas,an appropriate threshold is set to segment the sinogram to a binary image, the projection addresses of the rays penetrating the object are got based on the segmentation results and then the coordinate of COR for projection is obtained by calculating the mean of these projection addresses. The experimental results show that the method can measure the COR precisely and can improve the reconstructed image quality greatly.Moreover, the repeated measurement accuracy is less than 0.08 pixel. Compared with the existing methods, this method is simple and robustness and can meet the practical engineering requirements.