Solar water heater (PATS) is a water heating device whose source of energy comes from the sun. PATs that use water as heat stores have a deficiency of low energy density. One way to increase energy density is by applying a sensible heat storage (SHS) storage system with latent heat storage (LHS) where the energy density is high. Recent research on paraffin wax applications as thermal energy storage media in PATS thermosyphon has not discussed about stratification. This research is interesting to develop in terms of enlarged energy storage volume and the need for an investigation of the thermal stratification that occurs. The purpose of this study is to experimentally review the thermal stratification in the tank on a PATS thermosyphon system containing PCM (phase change material) during the charging process. This research begins by positioning PATS in the outdor room and closing the collector and filling the water in the tank. Data collection is done by taking data of radiation intensity of sun, PCM and HTF (heat transfer fluid) during charging process. The data and analysis exercise begins by plotting the graph of the intensity of solar radiation and outside air temperature as well as the HTF and PCM graphs during the charging process. The thermal stratification is done by calculating the Richardson number. Thermal stratification has been formed in the PATS tank during the charging process. This situation is visually visible on a graph of HTF evolution where the water is below, the temperature is lower. Stratification is also indicated by Richardson number of 2.36 at the end of the charging process.