This paper presents a predictive model for the Negative Bias Temperature Instability (NBTI) of PMOS under both short term and long term operation. Based on the reaction-diffusion (R-D) mechanism, this model accurately captures the dependence of NBTI on the oxide thickness (tox), the diffusing species (H or H2) and other key transistor and design parameters. In addition, we derive a closed form expression for the threshold voltage change (ΔVth) under multiple cycle dynamic operation. Model accuracy and efficiency were verified with 90-nm experimental and simulation data. We further investigated the impact of NBTI on representative digital circuits.