The characteristics of critical spontaneous ignition of granulated activated carbon were investiga-ted at a constant ambiemt temperature and sinusoidally varying ambient temperatures. At a constant ambient temperature, but larger vessels yielded the lower critical spontaneous ignition temperature. The apparent activation energy calculated by the Frank-Kamenetskii’s thermal ignition theory was 38.82(kcal/mol). At sinu-soidally varying ambient temperatures, the critical spontaneous ignition temperature was generally lower than that measured at the constant ambient temperature. For the amplitudes 5℃ and 10℃, a period of 1hour showed the minimum the of the critical spontaneous ignition temperature, and for the amplitude of 15℃, a period of 2 hours showed the minimum value of the critical spontaneous ignition temperature. The Frank-Kamenetskii’s δ_c(ε,ω) was determined from the results.