Thermal transient fatigue tests of ten pipe models with SUS304 steel and 21/4Cr-1Mo steel dissimilar joints were performed. Transient temperature distributions and thermal stresses caused by sudden a temperature change of the internal fluid were analyzed using FEM. Using these data, we evaluated the potential margin of the allowable number of thermal transient cycles according to the ASME Code Case N-47 and the elevated temperature structural design guide by the Power Reactor and Nuclear Fuel Development Corporation. The results are as follows: (1) The sequence of crack initiation of various parts of the test models could be well predicted by the analysis. (2) The buttering boundary should be kept suffiently apart from any discontinuity caused by counter-bore machining. (3) A sufficient safety margin for initiation of a crack 1mm in depth is observed and compared with the result based on the design rule by PNC.