As an important resolution enhancement technology (RET), off-axis illumination (OAI) can not only improve lithography resolution, but also improve depth of focus (DOF) to some degree. Finding a best OAI mode to improve lithography performance for specific mask pattern is the main research content. The best OAI mode is obtained through optimum design method, using the steepest descent algorithm as the optimum method, and using process window as the evaluation function. The process window contains three information: image fidelity, exposure latitude and depth of focus. The three aspects are described by performance of aerial image, and the three functions are weighted to get a comprehensive evaluation function. The complex photoresist model is avoided, and the evaluation function value can be obtained quickly and accurately by the propsed method. The optimal OAI modes and actual process window are solved for different weights, and the results show that the actual performance of process window can be reflected by evaluation function described through aerial image, and the performance of process window can be improved greatly by using the optimal source obtained by selecting right weight.