This paper investigates the dependence of current and voltage characterizations on stress time in deep submicron PMOSFET's before and after negative bias temperature (NBT) stress, we mainly focus on the threshold voltage shift under NBT stress. It is experimentally demonstrated that the electrochemical reactions at the interface between gate oxide and substrate and the diffusion of hydrogen related species in the oxide are the major causes of the NBT in PMOSFET's. PMOSFET degradation caused by NBT depends on the balance of reactionlimited and diffusionlimited mechanisms.