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Temperature fluctuation or thermal striping are considered in the hot and cold water mixing tees under plant piping systems, for example residual heat removal system of light water reactor (LWR). In order to draft the Japan Society of Mechanical Engineers (JSME) code concerning high-cycle thermal tatigue of piping systems, studies of thermal striping in the mixing tees has been conducted. Visualization tests and water temperature fluctuation measured tests were carried out in three type mixing tees. Large temperature fluctuation amplitude, ΔTpp=0.9∿1(ΔTrms=0.2∿0.3), occurred at near the junction of mixing tees in Type-A and Type-B. And the distance downstream from the tee junction increased, the fluid temperature fluctuation decreased. Flow pattern and temperature fluctuation almost depend on the velocity ratio. The effect of the main flow velocity was considered to be small. From these experimental results, temperature fluctuation reduction constant (β) characteristics (design chart(1)) were evaluated in several type mixing tees under velocity ratio and main velocity. The effects of upstream piping elements, elbow, valve and diffuser, were small. So piping element compensate constants (η) are about 1.