In a branch pipe with closed end, the cavity flow penetrates into the branch pipe from the main loop and the thermal boundary layer occurs because the cavity flow is a hot fluid, but the heat removal causes the colder fluid in the branch pipe. It is possible for the thermal stratification to affect the structural integrity. Therefore, it is necessary to establish the pipe design standard to suppress thermal fatigue. The pipe design standard consists of the maximum penetration depth L_<sv> and the minimum penetration depth L_<sh>. First of all, in order to establish the evaluation method for L_<sh>, the visualization test and the temperature fluctuation test were carried out. Theoretical formula of thermal stratification was introduced from the heat balance model. Using this model, the empirical equation was obtained from the map of fluid temperature fluctuation. The method can predict the vortex penetration depth by cavity flow in the horizontal branch pipe.