Numerous pipes branch off from main pipe in power plant facilities. Main flow initiates a cavity flow in the downward branch pipe with a closed end and thermally stratified layer may form in the branch pipe if there is significant temperature difference in main and branch fluid. Fluctuation of thermally stratified layer may initiate thermal fatigue crack in the branch pipe. Flow visualization experiment was conducted to understand detailed behavior of the layer, for vertical branch pipe with a closed end. The penetration length of main flow in the branch pipe was measured for various main flow velocities. The velocity distributions in the penetrating flow region were obtained by a LDV method. The temperature fluctuation near the thermally stratified layer was measured. Through these experiments it was found that; (1) the driving force of the fluctuations in the branch pipe was not due to thermal phenomena but fluctuation of the spiral flow; (2) the flow regime in the second region was simple structured forced vortex with constant angular velocity, (3) steady thermally stratified layer did not fluctuate periodically in the vertical pipe.