The pyrolysis kinetics of Huadian oil shale, spent oil shale obtained from near-critical water extraction experiments and their mixtures were investigated using thermogravimetric analysis (TGA). Experiments were performed at four different heating rates of 2, 10, 20 and 50 K min(-1) from ambient temperature to 850 degrees C at under atmospheric pressure and nitrogen flux. The results demonstrated that the thermal decomposition of oil shale, spent oil shale and their mixtures involved three degradation steps. The temperature of the maximum degradation rate (T-max) of the oil shale and mixtures shifted toward higher temperature with the increase of heating rate. The extent of interactions during co-pyrolysis was confirmed by comparing the experimental result with the theoretical one. Different thermogravimetric data were analyzed by integral method. According to the pyrolysis kinetic analysis, the apparent activation energy of spent oil shale and mixtures was lower than that of oil shale. The values of the obtained apparent activation energy of the mixtures decreased with an increasing spent oil shale fraction because of the decreasing of the macromolecular organic matters which needed much energy during the thermal decomposition. The results are reasonable to conclude that the spent oil shale was helpful for the degradation of oil shale in the mixture. (C) 2012 Elsevier B.V. All rights reserved.