The mechanism of negative bias temperature instability (NBTI) degradation in ultra deep submicron PMOSFET's is investigated. We mainly focus on the threshold voltage shift under subsequent positive bias temperature (PBT) stress after the preceding NBT. It's experimentally demonstrated that trapped holes from inversion channel and the diffusion of hydrogen molecules in the gate oxide are the major causes of NBTI degradation in PMOSFET's. When the condition is switched to PBT stress the trapped holes can be rapidly detrapped, but only a part of hydrogen molecules can diffuse back to the interface of gate oxide and substrate and repassivate silicon dangling bond, this is responsible for the threshold voltage being only partially recovered during PBT annealing.