Abstract: The effect of temperature and humidity on the rate of discoloration and degradation of vitamin K₃ powder was studied via accelerated test under isothermal, constant humidity and dark conditions. The photodegradation of vitamin K₃ powder was studied under room temperature and constant humidity. The changes of color and concentration of vitamin K₃ were monitored by diffuse reflectance spectroscopy and high performance liquid chromatography, respectively. At temperatures 50～90℃ and relative humidities 50～93%, the rate equation of discoloration is—dR/dt=k_RR, that of degradation is d (c_o—c)/dt=k_C (c_o—c)^1/2, and that of photodegradation is—dR/dt=k_p(R_o—R)^-1, where R_o, and R are initial relative reflectance and relative reflectance at time t, respectively, c_o and c are initial concentration and concentration at time t, respectively, k_R, k_C and k_p are apparent rate constants of discoloration, degradation and photodegradation, respectively. The general equation of the relationship between the apparent rate constant k, absolute temperature T and relative humidity RH is k=k′exp (—E_a/RT)exp (m′RH), where E_A is apparent activation energy, k′ and m′ are constants. Within the ranges of experimental temperatures and humidities, the average apparent activation energies of discoloration and degradation are 105.51 and 130.72 kJ/mol, respectively.