In this research, thermal decomposition of Huadian oil shale was studied by thermogravimetry (TG/DTG) at different heating rates in nitrogen environment. Apparent activation energy of the pyrolysis reaction for three samples has been determined at four heating rates (10, 20, 40, 100 degrees C/min) using Friedman's isoconversional method. Furthermore, the Sestak's complex mechanism was adopted to address mechanism of oil shale pyrolysis. The results showed that activation energy changes only slightly despite the fact that it is not a constant. In the conversion range of 0.1-0.9, the mean value of activation energies for samples OS1, OS2 and OS3 is 237.6, 249.61 and 242.67 kJ.mol(-1), and their standard deviations are 2.3, 3.86 and 4.61, respectively.