The high temperature sulfation of CaO with SO2 has been investigated by TG method under vacuum. Experimental data indicated that the sulfation process was a two-stage reaction, a very fast surface reaction with an activation energy of 36.7kJ/mol followed by a product-layer diffusion controlled reaction with an activation energy of 75.2kJ/mol. The initial period was about 7s. This process of sulfation was affected by limestone type, micro structure, particle size and temperature, while the SO2 concentration hardly affected the sulfation. A 59% CaO conversion was achieved in 30s at 1000℃ and 1×102pa.