A three‐dimensional (3D) photolithography simulator was developed which is composed of the two‐dimensional (2D) mask aerial image simulator, simulator for 2D intensity pattern in aligner (STIPAL), and the 3D resist image simulator, resist process 3D simulator (RESPROT). STIPAL can calculate the mask aerial image in projection printing considering lens aberrations and reticle defects which are very important factors influencing submicron pattern transfer, in addition to the parameters of lens NA (numerical aperture), source wavelength λ, partial coherency σ and defocusing. RESPROT can simulate the 3D resist image in a conventional resist process and contrast enhancement lithography (CEL) using the mask aerial image data calculated by STIPAL. Resist pattern printability or fidelity in the submicron process can be analyzed by these simulators. Resist linewidth shifts caused by reticle defects and lens aberrations were analyzed. Effects of a dark defect (a surplus pattern) or a clear defect (a lack of pattern) on the resist linewidth shifts are almost the same when defocusing and/or lens aberration do not occur. When defocusing and/or lens aberration occur, however the resist linewidth shift caused by a dark defect is larger than that caused by a clear defect. A high resolution photoresist and CEL can decrease the resist linewidth shift caused by mask defects. Further, CEL can decrease oval distortion of the submicron contact hole pattern caused by lens aberration.