Negative bias temperature instability (NBTI) has come to the forefront of critical reliability phenomena in advanced CMOS technology. In this paper, we propose a fast and accurate PMOS NBTI model, in which the temperature variation and the ratio of active to standby time are considered in both stress and relaxation phases. A PMOS Vth degradation model and a digital circuits? temporal performance degradation estimation method are developed based on our PMOS NBTI model. The simulation results show that: 1) our dynamic NBTI model without temperature variation is as accurate as previous models, the error is less than 2.3%; 2) the analysis error of PMOSVth degradation may reach up to 52.6% without considering temperature variation; 3) for ISCAS85 benchmark circuits, the error of worst case performance degradation analysis is about on average 52.0%; 4) the ratio of active to standby time has a considerable impact during the performance degradation analysis.