The main goal of this present study is to investigate the effects of the application of PCM and Nano-enhanced PCM as wallboard on the thermal behavior of a room. For this purpose, a room was modeled in two dimensions under Tehran's summer weather conditions through computational fluid dynamics (CFD). The effect of using the PCM as a wallboard in the southern wall in both pure and enhanced with nanoparticles was investigated. The indoor temperature, the wall surface temperature, and the interior wall heat flux, in both cases, were reported and compared. At the end of this study, the acquired results were compared with the pre-modified room, and thermal improvement was reported. The results indicate that the use of solid nanoparticles in PCM reduces the energy consumption of air-conditioning system by 7.4% compared to the conventional room. In the case of the Nano-enhanced PCM wallboard, the room has better thermal performance than the pure PCM, with 4.37% more energy storage, about 0.273 reductions in temperature decrement factor, and a 21.6 min increase in the time delay to peak temperature. Compared to the conventional room and room with pure PCM, the room’s temperature fluctuation, modified by Nano-enhanced PCM, reduces by 52% and 31%, respectively. This study's obtained results could help the researchers and designers have a more appropriate PCM selection for building ventilation system applications.