Negative Bias Temperature Instability (NBTI) of p-channel power MOSFETs is a primary concern in gate oxide degradation. Instead of a qualitative conclusion, predicting the degradation in the on-state resistance during long-term operation is significant. This paper evaluates the degradation of each component of the on-state resistance under NBTI stress. By dividing the on-state resistance into two parts based on whether it is affected by NBTI, we obtain a first-order linear model form to model the on-state resistance. This study can support the physical modelling and the remaining useful life prediction of the on-state resistance in long-term operation.