Negative Bias Temperature Instability (NBTI) is one of the major reliability problems in advanced technologies. NBTI causes threshold voltage degradation in a PMOS transistor which is biased to negative voltage. In an SRAM cell, due to NBTI, threshold voltage degrades in the load PMOS transistors. The degradation has the impact on Static Noise Margin (SNM), which is a measure of read stability of a 6-T SRAM cell. In this paper, the authors discuss the relationship between NBTI degradation in an SRAM cell and the signal probability.