1. A simulation method in a lithography process, comprising the steps of: obtaining an aerial image by inputting exposure apparatus conditions and a mask layout in a resist process for I-lines; regarding said aerial image the same as an acid distribution being produced from a resist in an exposure process, and obtaining a diffused aerial image in consideration of diffusion of said acid in a post expose bake process; applying said diffused aerial image to a threshold model, thereby modeling an expected resist pattern size.