In the present study, preparation and thermal characterization of the microencapsulated lauric acid (LA) and paraffin (Pa) with a poly(styrene) shell was carried out using an emulsion polymerization technique for the first time. The optimization of the microencapsulation process of the LA and Pa as phase change materials (PCM) was investigated using response surface methodology (RSM). The quadratic polynomial regression model was applied for predicting microencapsulation ratio. The maximum achieved value of microencapsulation ratio under the optimal conditions was 87.7% and 91.64% for Pa and LA, respectively. The optimal independent variables using response surface methodology were PA/styrene (St) mass ratio of 0.39, emulsifier (SDS)/ St mass ratio of 0.035 and the stirring rate of 1500 rpm in the microencapsulation process of PA. For the microencapsulation process of LA, the optimal variables determined LA/St mass ratio of 0.42, SDS/St mass ratio of 0.01, stirring rate of 1076 rpm and the temperature of 55 o C using RSM. Under the optimal conditions, the average diameter of Pa and LA microcapsules is about 18.3 ?m and 1.32 ?m, respectively. Thermogravimeric analysis (TGA) showed that thermal stability of microencapsulated samples are higher than that for pure LA and Pa samples. The results showed that obtained microcapsules with the maximum microencapsulation ratio can be considered to have good potential for energy storage. After determination of the optimal conditions, the effect of presence of three kinds of nanoparticle of MgO, TiO 2 and graphite with mass ratios of 3, 5, 7 and 10% on the thermal properties of the Pa and LA microcapsules was investigated. The microencapsulation ratio of LA increases with amount of MgO and TiO 2 . However, the microencapsulation ratio was reduced with increasing the mass ratio of nano graphite. MgO and TiO 2 nanoparticles were inlayed in paraffin microcapsules by surface modification. The microencapsulation ratio of Pa decreased by increasing the mass ratio of these nanoparticles. It could be due to the difference in the structure of Pa and LA. The diameter of the paraffin microcapsules with nanoparticles is larger than that of the LA microcapsules with similar nanoparticles. The LA and Pa microcapsules with TiO 2 nanoparticles have the maximum microencapsulation ratio compared to other types of nanoparticles. Scanning electron microscopy (SEM) images showed that microcapsules obtained containing TiO 2 were spherical with smooth surface and narrow particle size distribution. The thermal stability and thermal conductivity coefficient for the pure phase change materials, obtained microcapsules with and without nanoparticles were investigated. The thermal stability improved with increasing mass ratio of the nanoparticles, no considerably. Thermal conductivity of the microcapsules was increased with addition of nanoparticles. The results show that LA and Pa microcapsules with TiO 2 nanoparticle have higher thermal stability than other nanoparticles. Thermal performance of the obtained samples was evaluated in a designed experimental setup. The results show that thelopeoftemperature-time in heating curveof the LA samples is more thaa samples. It is verified by the higher thermal conductivity coefficient of LA samples. Also, the slope of heating curves of the LA microcapsules containing MgO, TiO 2 , graphite, and microcapsules without nanoparticle, respectively increases in comparison to the slope of heating curve of the pure LA, due to higher thermal conductivity coefficient of corresponding samples. The Pa and LA microcapsules with MgO nanoparticle have the most thermal conductivity coefficient and attain the highest temperature stability in the heating process. According to the thermal conductivity enhancement of the microcapsules in comparison with the pure samples, the start of melting and solidification process was considerably accelerated in the heating and cooling process, respectively. Keywords: Phase Change Material, Microencapsulation, Polymerization, Optimization, Nanoparticles, Thermal conductivity coefficient, Thermal Performance.