The thermal performance of hybrid hollow plaster panels (HHPPs) was analyzed using the amount of phase change material (PCM) injection as a variable according to the size of the hollow area. This study focuses on n-octadecane, an organic PCM that is used for storing latent heat during the phase change range and to improve thermal transmittance using exfoliated graphite nanoplatelets (xGnPs), which have a high thermal conductivity. When xGnP is applied to n-octadecane, the thermal conductivity improves by 225%, and it is confirmed that the thermal storage or release of the phase change material is an active reaction. The thereto-physical properties of the xGnP and n-octadecane composites were analyzed using a thermal conductivity analyzer (TCi) and differential scanning calorimetry (DSC). The thermal stability of PCM was analyzed over a long duration of 10,000 thermal cycles. The thermal performance of the PCM/plaster composite panel using the dynamic heat transfer device was determined. The peak temperature through the HHPP significantly reduced by 3.8 T in an internal room, and the time-lag effect was confirmed to be 1.56 h. The results indicate that up to 36.6 J/m(2) of thermal energy was stored in the 26-Px/O, corresponding to approximately 247% of the available thermal energy of the reference panel.