Thermal transient water tests were carried out using a simplified 1/10-scale of pool type Liquid Metal Fast Breeder Reactor to examine the characteristics of thermal stratification during mixed circulation. Various experiments as the conditions of primary flow coastdown and decay power were varied. Thermal stratification may cause significant thermal fatigue to the wall of the hot plenum and the internal components. The main purpose of this work is to understand the temperature time gradient at the thermal interface in the hot pool. A correlation is developed for the temperature time gradient at the interface in terms of Reynold number and Richardson number.