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Phase change material (PCM) is known as a material that has high energy density. The integration between water and PCM in a solar water heater (SWH) system has a potential method to increase the capacity of thermal energy storage. One of important parameter in a SWH tank is a thermal stratification. This parameter can affect thermal efficiency of a SWH system. The aim of this paper is to analyze the thermal stratification of water in a SWH tank containing PCM. PCM is inserted in a cylindrical capsule. The bundles of capsules are arranged in a SWH tank. An outdoor experiment is conducted on a SWH system during charging process. The intensity of solar radiation, water and PCM temperature are recorded. The resulting data was used to make the temperature evolution of water. Stratification analysis was performed using the Richardson number calculation. Based on the evolution of water temperature, it appears that there is a temperature difference in each layer of water. The results also show that the Richardson number is greater than 1. This result confirms that thermal stratification has been formed in a tank. The addition of PCM in a SWH tank is able to maintain thermal stratification.