Granular γ-Fe2O3 powders, acicular γ-Fe2O3, and CoFe–γ-Fe2O3 powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants K E of granular γ-Fe2O3 powders at different temperatures are obtained by using the law of approach to saturation (LATS). KE values of acicular γ-Fe2O3 and CoFe–γ-Fe2O3 powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of KE with temperature for granular γ-Fe2O3 is about the same as that of shape magnetic anisotropy Ksh. Fluctuation field Hf and activation volume Vf of samples are measured. A theoretical expression of Vf is derived. For granular γ-Fe2O3 powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe2O3 powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe2O3 at different temperatures is close to homogeneous rotation.