A new technique for measuring the aberrations of a lithographic projection system in situ is proposed, in which the basic theory used to measure the spherical aberration, coma and astigmatism of the projection system with special measurement mark is analyzed in detail, and the method to calculated the image displacement caused by aberration through aligned position coordinate is presented. Through experiments, the aberration parameters, such as spherical aberration, coma and astigmatism of the projection system are obtained with high accuracy by the technique. Taking into account of the influences of process factors, e.g. resist process, on the image displacement induced by aberration, the new technique avoids the dependence of coma in-situ measurement based on wafer exposure on the image parameters, such as defocus and image tilt.