Abstract: Kaolin filled UHMWPE composites were prepared by melt mixing and polymerization respectively. Their thermal properties, crystallizability, aggregate structure and tribological behaviors sliding against 45 steel were tested and studied thoroughly, and the relationship between the tribological behavior and the structure was analyzed. It was found that the polymerization-filled composites have much better tribological properties than the mixing-filled ones with the same components, and the relevant properties are closely related to the reinforcement of kaolin particles dispersed evenly, the combining strength of the interface and the crystal patterns.