Isotherm characteristics of red pepper powder and the effect of temperature and water activity (Aw) on its color change were investigated. Monolayer moisture contents of red pepper powder decreased from 0.1218 to 0.0912 g water/g solid with increasing temperature from 25 to $50^{circ}C$. The color change of red pepper powder was greatly dependent on temperature and Aw. As temperature and Aw increased, red color of pepper powder increasingly faded out to become brown and tarnish black, which is mainly attributed to the degradation of carotenoid pigments and development of browning compounds. Color parameters such as Hunter-L, a, b values and other color functions as well as browning index and ASTA color values represent color changes of red pepper powder as influenced by temperature and Aw.