In this paper, an accurate model of resistive drift region in laterally diffused metal oxide semiconductor (LDMOS) is presented. We have improved the dynamically varying depletion width model at the drift/substrate junction, by considering both the drain and substrate bias effects. The both bias condition causes the resistance modification of the high resistive drift region for the reduced-surface-field structure (RESURF). In addition, new expansion-effect model is developed by considering the substrate bias effect. The developed models have been implemented into Hiroshima University starc insulated gate field effect transistor model for high voltage (HiSIM-HV) and it is proved that the models reproduce measured direct current (DC) characteristics for the wide variety of the substrate bias variation.