The main objective of this study to evaluate the performance of phase change material (PCM) in smoothing outlet temperature of double tube heat exchanger subjected to fluctuating inlet temperature. A computational fluid dynamics (CFD) model is formulated, validated and utilized to investigate the conjugate heat transfer between heat transfer fluid (HTF) and PCM inside the heat exchanger. Three organic-based PCMs, i.e. n-octadecane (C18H38), n-eicosane (C20H42) and myristic acid (CH3(CH2)12COOH), were investigated to study the importance of PCM melting temperature on the performance of the PCM based thermal capacitor. The effect of heat transfer fluid inlet Reynolds was evaluated as well. The result confirms the potential usage of PCM as smoothing device (thermal capacitor). Moreover, the outlet temperature fluctuation was found to decrease as the inlet Reynolds number increases. Selecting PCM with appropriate melting points is crucial in ensuring successful application of PCM as thermal capacitor for specific application.