In this work, thermal cracking inside an oil shale sample under different temperatures was tested using a micro-CT system. The critical temperature was determined at which sharp thermal cracking in the oil shale sample occurs. It was found that the critical temperature of Chinese Fushun oil shale is 350 degrees C. After reaching 350 degrees C, all sites of the sample reveal fissures whose number, length and width increase dramatically, resulting in the formation of an enormous network of fissures. A high-temperature rock-permeability testing machine was used to test the changes in permeability at different temperatures. The results similar to those of thermal cracking were obtained. Therefore, thermal cracking can be regarded as the decisive factor to affect the changes of permeability in oil shale particle.