Abstract: The effect of temperature and humidity on the discoloration and degradation of vitamin C powder was studied via accelerated test under isothermal and constant humidity conditions. The changes of color and concentration of vitamin C were monitored by diffuse reflectance spectroscopy and iodimetry respectively. Under the condition of relative humidity less than the critical relative humidity, the apparent rate constant of discoloration k_c of vitamin C powder is: k_c=kéxp (-Ea/RT)exp (m'RH ), where k' and m'are constants, RH is relative humidity and E_a is activation energy, when RH<70%, E_a=104.33 kJ·mol^-1. The degradation curve of vitamin C powder under various temperatures and humidities can be expressed by C=N/(1+Btⁿ), the rate equation of degradation of vitamin C powder is:-d C/dt =k_dC^1+1/n (N-C)^1-1/n, where k_d is the apparent rate constant of degradation, B and n are constants, N is the initial content of vitamin C and C is the content of vitamin C at time t. When RH are 97,93, and 79.5%, Ea are 86.87,88.22 and 202.64kJ·mol^-1, respectively.