A fundamental investigation on a key vortical structure in film cooling flow, which is called counter-rotating vortex pair (CRVP), has been performed. Traditionally, the coolant’s momentum flux ratio is thought as the most critical parameter on film cooling effectiveness, which is the index of film cooling performance, and this performance is also influenced notably by CRVP. About the sources of CRVP, the in-tube vortex, the in-tube boundary layer vorticity, the jet/mainstream interaction effect, alone or combined, are proposed as the main source in the literature.  A numerical approach was applied in present study. By simulating a general inclined cylindrical cooling hole on a flat plate (the baseline case), the CRVP was visualized as well as the in-tube vortex.  Another case, which is identical with the baseline except the boundary condition of the in-tube wall was set as free-slip to isolate its boundary layer effect, was simulated for comparing. Their comparisons have clarified that the jet/mainstream interaction is the only essential source of CRVP. Through further analyzing its mechanism, CRVP was found to be a pair of x direction (mainstream wise direction) vortices. Hence, the velocity gradients -v/z and w/y were the promoters of CRVP. Applying this mechanism, a new scheme named nozzle scheme was designed to control the CRVP intensity and isolate the overall momentum flux ratio Iov, a parameter used in literature. Analysis of the effects of CRVP intensity and momentum flux ratio on film cooling effectiveness has demonstrated that the CRVP intensity, instead of the momentum flux ratio, was the most critical factor governing the film cooling performance.