This study is focused on development of pipe design standard to suppress thermal fatigue, and the developed model evaluates Lsh defined as penetration length of cavity flow into a branch pipe with closed end which consists of a vertical and a horizontal pipe connecting to a main loop with fluid flow. This evaluation model is applied to phenomenon that fluid temperature in a main loop is higher than that in a branch pipe and an atmosphere because thermal stratification layer appears due to temperature differences between cavity flow and fluid in a branch pipe under that condition, and thermal stratification affects the structural integrity. In order to establish the evaluation method for Lsh, the visualization tests and the high temperature and high pressure experiments were carried out for the branch pipe consist of a vertical and a horizontal pipe connected with an elbow. The evaluation method can estimate the penetration length by the cavity flow in the several types of the branch pipes within 20% accuracy.