Various studies have been conducted to use solar energy, one of renewable energy sources under the CO2 reduction policy. Such a research about solar energy storage with salts or other heating medium is also being carried out in the US and Europe actively. We also have been developing the leading technology to increase the storage efficiency, and proved that stratification of thermal medium raise the efficiency. Accordingly, we are focusing on the method to increase the stratification by applying diffuser and varying the diffuser shapes. In this experiment, we investigated the effect of the plate diffuser on the stratification of the water as thermal medium in a visible tank.