As critical dimension shrinks, the effect of coma aberration on the performance of modern lithographic tools has become increasingly obvious. So, research on high-accuracy in-situ measurement of coma aberration is necessary. In this paper, a new method to measure coma aberration in projection system is proposed. The principle of the method is described in detail. The coma-induced image displacements are simulated at different defocus positions by simulation program PROLITH. The sensitivity coefficients of coma aberration are calculated. The advantage of the method is that the measurement accuracy of coma aberration can increase approximately by 25% compared with commonly used TAMIS method.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.