To reveal the hot carrier induced degradation (HCI) mechanism of 40 V lateral-diffused NMOS, TCAD simulation was firstly performed to predict the impact ionization strength and distribution along the lateral direction. IV characteristic curve measurements were subsequently performed to monitor Ids and Ron shift to offer a preliminary explanation towards the physical mechanism of degradation. Meanwhile, a strong positive correlation between Isub and Ron degradation was observed experimentally. Hence, Isub could be a promising index for HCI immunity evaluation in predicting time-to-failure (TTF) upon Ron degradation. Charge pumping technique was also performed to precisely identify the location and quantity of interface states during DC electric stressing. A comprehensive investigation of hot carrier degradation mechanism was clarified and reported in this paper. Eventually, hot carrier safe-operation-area (SOA) is defined for evaluating the performance of the LDMOS transistors and for circuit designers' reference.