The effect of static negative bias temperature instability stress on p-channel power metal-oxide-semiconductor field-effect transistor (MOSFET) is investigated by experiment and simulation. The time evolution of the negative bias temperature instability degradation presents the trend which follows the reaction-diffusion (R-D) theory on the exaggerated time scale. A flat-roof section is observed under the varying stress condition, which can be considered as the dynamic equilibrium phase through the simulation verification based on the R-D model. The analysis of the simulated results also provides the explanation for the difference in the time duration of the dynamic equilibrium phase under the condition of varying stress voltage.