This study shows the evaluation method about the thermal performance of the building thermal storage materials applying Phase Change Material. Firstly, the authors invented the interior building materials mixing micro-capsuled PCM (mPCM), and measured the basic thermal performance in a constant temperature chamber. PCM is expected to reduce the energy consumption of the heating and cooling system, providing a comfortable environment for residents.<br> We focused on mPCM, which includes n-paraffin at melting points of 20°C and 25°C, around the normal setting temperature of the heating and cooling system. It is estimated that mPCM's are effective for passive thermal storage use all year round. The mPCM contains n-Heptadecane and n-Octadecan. The base ingredient of the interior building material is Ready mix Gypsum Plaster (RGP). The PCM boards are made of RGP, mPCM and water. In addition, another PCM board mixing the two different melting point mPCM (20°C, 25°C: each 50wt.%) was developed and the basic thermal performance was measured. The thermal storage rate and Mass density of the PCM board were revealed to be dependent on the content of mPCM (Table1 and Table2). As a result, the mixing rate of the mPCM proved to be proportional to the comparably high thermal storage performance (Fig. 5, Fig6, and Fig7).<br> On the other hand, the thermal storage rate of the PCM boards mixing two different melting points of mPCM (20°C, 25°C: each 50wt.%) turns out to have an intermediate value between two melting points of PCM boards (Fig. 8). Without changing the mixing ratio of the encapsulated n-paraffin in the core portion, potential capability of changing the apparent melting point is suggested from this experimental result.<br> Second, the formulation of the apparent specific heat was proposed based on the solid-liquid phase of each PCM board. In previous studies, the method of distribution of the latent heat in the vicinity of the melting point has been shown with low calculation accuracy. Since the specific heat of mPCM varies with temperature, it is difficult to accurately predict the heat flux and temperature fluctuation based on previous unsteady methods of numerical calculation in unsteady state.<br> Therefore, the authors defined the specific heat of PCM board as a function of temperature by harmonic analysis (Fig. 10, Fig. 11 and Table3). The result of measurement on thermal storage is approximated in a sixth order polynomial and that equation is differentiated. It was confirmed that the results of the harmonic analysis were accurately approximate to the experimental results at the temperature range 10-40°C. Furthermore, the surface temperature of PCM boards were measured by these heating experiments and the experimental data was used for the numerical calculation of the influx and efflux as a boundary condition (Fig. 9). The measured results of heat flux and thermal storage were accurately approximated by numerical calculation (Fig. 12 and Fig. 13).<br> Finally, the room temperature fluctuation with PCM board was estimated by unsteady thermal calculation applying the method mentioned above. The accuracy of the simulation was examined by comparing the calculated results with the experimental results. Room temperature, the outdoor temperature and solar radiation were measured in the experimental modules in winter (Fig. 14 and Fig15). Calculated results have clarified that it is possible to predict measurements in solid-liquid phase (Fig. 17 and Fig. 18). For these reasons, the formulations of specific heat in this study were confirmed to be usable to predict the temperature fluctuation and energy consumption of a room with PCM boards. Further, room temperature difference were observed during intermittent heating (Fig. 19).