This paper applies a theoretical process model developed by Dilletal. to the polychromatic exposure systems. Exposure and development of a positive photoresist are treated with computer simulation for both contact printing and projection printing. Optical interference within resist film and the effect of it on the development condition are investigated theoretically. The results for the two printing systems are compared in order to clarify the differences between contact exposure and projection exposure. It was made clear that (1) the optical interference still has a considerable influence on the condition imposed on the photolithographic process even in the polychromatic exposure environment and (2) the projection exposure with two spectral lines is more sensitive to such parameters as the thicknesses of resist film and the underlying oxide layer than the contact exposure with three spectral lines.