Among various methods of thermal energy storage, latent heat energy storage is more widely used than sensible and chemical energy. Phase change materials (PCMs) have been extensively used as latent heat storage materials in a variety of fields including solar energy utilization, industrial heat loss recovery, thermal insulation in construction, textile industries and so on. Fatty acids have superior advantages over other PCMs because of their properties such as high enthalpy of latent heat, non- toxicity, non-corrosiveness, non-flammability. In the present work, lauric acid microcapsules were fabricated with polystyrene shell and their thermal properties were compared with pure lauric acid. Then, the effect of silica hydrophobic nanoparticles on the thermal properties of microcapsules has been investigated. In the first step, the lauric acid microcapsule with a polystyrene shell was made without the presence of nanoparticles and its thermal properties was analysied. In the next step, silica nanoparticles, at different weight percentages of 0.75%, 1.5% and 3%, were dispersed once in styrene as microcapsule shell and again in lauric acid as microcapsule core using ultrasonic waves, followed by encapsulation process through emulsion polymerization method. DSC, FT-IR, SEM and XRD analysis, weightless percentage at high temperature, variations in thermal diffusivity of samples and thermal performance of the samples were investigated and compared for all three types of products. Results showed that lauric acid microencapsulated with a considerable encapsulation efficiency (about 78%) and the thermal stability of the resulting micro-capsules was 36% more than that of pure lauric acid. On the other hand, the presence of hydrophobic silica nanoparticles, whether in lauric acid as a core or in polystyrene as a shell, made no chemical bond with the material; however, it disrupted the structure of the microcapsules according to SEM images and converts them into porous micron pores of polystyrene containing lauric acid and nanoparticles. The encapsulation efficiency and thermal stability of the samples containing silica nanoparticles were much lower than the lauric acid microcapsules due to the decrease in the mass fraction of lauric acid limited in polystyrene, however, the encapsulation ratio for the samples containing nanoparticles in the shell was higher than those containing nanoparticles in the core. This may be due to the higher porosity of the structure produced in the samples containing nanoparticles in the shell and the limited of a higher percentage of lauric acid in it. Thermal diffusivity was also 27% higher for pure microcapsule samples than pure lauric acid, but this ratio was significantly lower for samples containing silica nanoparticles. As the percentage of silica nanoparticles in the samples increased, the thermal diffusivity decreased. By evaluating the performance of the while placed in an oil bath at 80 ° C, it was observed that the lauric acid microcapsules without nanoparticles began to melt earlier than all samples containing silica nanoparticles and even pure lauric acid. Keywords: Microencapsulation, PCM, Lauric Acid, Silica Nanoparticles, Thermal Properties