Abstract: The thermal conductivity and diffusivity of loose coal can be affected by different factors, and they are important parameters to research the spontaneous combustion of coal. But for the same coal, the thermal conductivity and diffusivity are mainly related to particle size. They can be measured meanwhile through the modified transient plane source method. Through the measurement of loose coal with small particle size and mixed loose coal, the change laws were got. The results show that, with the increasing of particle size, thermal conductivity decreases gradually; thermal diffusivity decreases first and then increases. When the weighted size of loose coal is the same, the thermal conductivity and diffusivity of mixed loose coal are affected by loose coal with smaller particle size.