The buildings are responsible to 40% of the global greenhouse gas emission. To tackle the climate change, the development of sustainable building is urgent. Phase Change Material (PCM) as a potential material was suggested to integrate in building envelope to maintain a constant room temperature. This paper investigated the thermal performance of the PCM in building envelope at tropics and optimizes the cooling effect by manipulating melting temperature, thickness, position, and the type of PCM. ANSYS FLUENT software is used to model the 2D cross-sectional area of the steel roof and brick wall. With the input of 2 normal days outdoor temperature, the results of simulated indoor surface temperature are compared with the control set to determine the optimum setting while monitoring on its cost-effectiveness. The results showed that the optimum melting temperature of PCM is 29 °C for brick wall application and 31 °C for steel roof application. In normal day, PCM can reduce the peak indoor surface temperature up to 4 °C. Although in the evening, PCM delays the time for the room to cool off, but for the normal warehouse operating hours this is not affecting the thermal comfortability of the workers.