This paper deals with the thermal performances of PCM/diatomite composites for energy saving. The PCM/diatomite composites were prepared by incorporating PCMs in the pores of diatomite to increase form stability of PCMs. In experiment, we used hexadecane and octadecane as PCM. The PCMs could be retained at 50wt% in the pores of the diatomite without leakage. Thermal efficiency was also analyzed through vacuum treatment during the preparation process. An optimal preparing method for 50wt% of PCM impregnation is proposed. Thermal properties of PCM/diatomite composites were determined using DSC, and were characterized by SEM and FTIR analysis. SEM results showed that the PCMs are well- infiltrated into the structure of diatomite. DSC analysis results showed that the latent heat capacities of PCM/diatomite composites were 50% the value of pure PCMs.