Optical proximity correction technique has been studied. The occurrence of proximity effect in the optical lithography is composed of an element caused by diffraction of light, which can be explained by aerial image simulation, and an element caused by resist process, in which acid diffusion is a major factor causing non-linearity. In the case of very thin resist, absorbed energy that generates the acid can be described by two-dimensional instead of three-dimensional distribution. Under this simple assumption, acid diffusion by post exposure bake is equivalent to the diffusion of aerial image, and chemical amplification can be analytically described also. Modified aerial image including diffusion and chemical amplification, we call it diffused aerial image, can give the information for patterning status directly. Therefore, diffused aerial image model can explain experimental results very well compared to the expectation by using aerial image only without loss of simplicity and calculation speed.