The current investigation clarifies the key factor on the film cooling effectiveness, which is the fundamental of the film cooling research. Conventionally, the momentum flux ratio is thought as the most critical factor on the film cooling effectiveness, and the counterrotating vortex pair (CRVP) also has significant impact. A new scheme named the nozzle scheme has been created to separate the momentum flux ratio and control the CRVP intensity. Three configurations of the nozzle scheme and the baseline have been simulated numerically under density ratio of 2 and blowing ratios that varied from 0.5 to 2. The results demonstrate that the CRVP intensity, instead of the momentum flux ratio, is the most critical factor governing the film-cooling effectiveness, and the mainstream direction component is its main component. Therefore, its two compositions, which are the corresponding velocity gradients, naturally become the key parameters considered in the design of new film cooling geometry.