As the semiconductor technology node comes to 14 nm and below, using bright-field exposure with negative tone development (NTD) has been a dominant lithographic solution for metal and contact layers, which has benefits of larger process windows and higher image contrasts than positive tone development (PTD). For PTD, a resist model is usually optional in source mask optimization (SMO) because optical models with aerial image blur can predict resist behaviors in most cases. However, NTD has much stronger resist effects, such as resist shrinkage and two-dimensional-effect-induced local stress. It has been suggested that the calibrated resist model is strongly required in the SMO of NTD process. We clarify this issue—the necessity of resist model in SMO for NTD process. First, we analyze the mismatch between simulation and experimental data when the aerial image blur is only used to simulate resist effects. Second, we present the calibration flow of resist model. Finally, we use the calibrated resist model to check the test pattern and run the SMO. The result demonstrates that the simulation data have the same tendency with experimental data, and the model has a good prediction on NTD resist behaviors under different conditions.