Both nominal and process window (PW) OPC models were calibrated against wafer data from single and multiple process conditions, respectively. Based on different definition for nominal process focus condition, different OPC models were obtained. The effects of nominal process focus definition on the model performance were explored by analyzing the focus parameters calibrated and the data fitting accuracy and prediction capabilities of the models for different process conditions. For both nominal and PW OPC models, the nominal process focus setting has no obvious influence on fitting accuracy for particular process, however, proper nominal process focus selection results in better PW OPC model for nominal process defined.