Recently internal combustion engine’s efficiency has been improved dramatically from customer’s demand that is to get car of good fuel efficiency. For this reason, customers are requesting the development of lightweight and highperformance heat exchanger. As part of this effort, research and experiments have been conducted on the thickness reduction of the radiator tube in order to satisfy the light weight of the heat exchanger. Thereby radiator tube thickness is a tendency to have become thinner and thinner. It is a great advantage in the design of lightweight radiator. However, it is possible to decrease in durability occurring material becomes thinner. The thickness of the material is determined to be the most important elements of the thermal cycle test. Particularly thermal expansion and contraction of the length of the tube is different from the conventional materials, It could be possible to adversely affect the thermal cycle durability life. This paper has been started to improve the radiator thermal cycle durability by changing shape of the header without changing the thickness of the radiator tube to compensate for thermal cycle durability that may be disadvantageous for thinning of the tube material.