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This paper presents an experimental and numerical investigation on the cooling performances of a flat endwall, which is made of super-alloy. The endwall passage is configured by two semi-profiled vanes. In the flat endwall, there are 34 cylindrical film holes located at the leading edge region, near the pressure side and upstream inlet, respectively. The experiments were carried out in the hot wind tunnel at the University of Science and Technology of China. Cooling characteristics were captured at different blowing ratios and mainstream temperatures by an infrared thermal image system. The experiments exhibited an interesting phenomenon, i.e. one can get a similar temperature contour at different mainstream and coolant temperatures but the same other conditions if using the maximal and minimal temperatures as the limit values in the contour scale. The experimental data were used to validate the numerical strategy. To demonstrate this similar phenomenon at the real condition of gas turbine operations, numerical simulations were conducted. The numerical results indicated that at the same blowing ratio and mainstream Reynolds number, when mainstream temperature varies from 1700K to 1950K but cooling air temperature keeps at 850K, or the cooling air temperature changes from 750K to 950K but the mainstream temperature keeps at 1700K, the temperature contours generated by the scale of nature maximal and minimal temperatures are very similar. Using linear relationships to fit these maximal and minimal temperatures in the contour scales, the maximal fitting error is less than 0.1%. Using the similar phenomenon and linear relationships, one can easily predict any cooling effect at the condition between two experimental states or two simulation conditions. Therefore this similarity is very useful for the experimental investigations into endwall cooling performances and the design of film cooling configurations.