CaO formed by decomposing CaCO3 at 1450 degrees C was ground in a vibrational mill. The variations of mechanochemical properties of the sample, such as surface area, crystal structure, surface free energy and reaction activity were studied. Inverse gas chromatography and reaction activity were studied. Inverse gas chromatography (IGC) was used in the analysis of surface free energy of the sample. It is concluded that the total surface free energy increases with grinding. The London dispersive component of the surface free energy increases continuously with grinding, while the polar component firstly increases with grinding, and then decreases and finally disappears, when grinding proceeds. The activation of CaO powder was studied by measuring the dissolving and hydration rates of CaO powder in 1 mol/l HCl and distilled water respectively. CaO ground for 200 h was reheated at different temperatures. With the recovery of lattice distortion and the recrystallization of the amorphous part, both the reaction activity and the surface free energy decrease, the polar component appears again.