A method of predicting photoresist patterns defined by a plurality of photomask patterns is described. The measurement data of photoresist patterns defined by patterns on a photomask that are arranged similar to the photomask patterns are provided. A physical optical kernel and a mathematical load kernel as a part of a Gaussian distribution function or other distribution function or as a combined function including a part of a Gaussian distribution function or other distribution function are provided. The optimal values of the parameters of the mathematical load kernel are determined by fitting the experiment data with a simulation based on the graphic data of the patterns on the photomask and the kernels. Photoresist patterns defined by the photomask patterns are simulated based on the graphic data of the photomask patterns, the physical optical kernel, and the mathematical load kernel with the optimal values of the parameters determined.