The main aim of this study is to prepare and characterize PCM/diatomite composites with impregnation method using unmodified and microwave modified diatomite. The effect of microwave modification of diatomite on composites was investigated. In this study, lauric acid (LA) was used as the phase change material and the prepared PCM/diatomite composites were analysed by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and laser diffraction particle size analyser. Thermal stability of composites was demonstrated by thermal resistance test. The results show that the composites prepared with microwave-modified diatomite have higher heat capacities than those prepared with unmodified diatomite. According to the DSC analysis, composites with modified diatomite have melting and freezing temperatures of 40.4 and 39.5 degrees C, respectively. These composites also have the latent heats of melting and freezing of 55.7 and -49.9 J/g, respectively. According to the analysis and thermal resistance test, it was seen that composites were prepared successfully. After the determination of better thermal energy storage properties of microwave-modified diatomite containing composites than unmodified diatomite containing composites, we recommend that these composites can be used for thermal energy storage applications.