The effect of internal crossflow in cooling channel on film cooling performance over a flat plate with a row of cylindrical film cooling holes is numerically investigated over a wide range of blowing ratios from 0.5 to 2.0 and crossflow ratios from 0 to 3.0. The results show that the internal coolant flow leads to a changed film cooling performance. By comparison to the baseline stagnant plenum coolant supply or no-crossflow case, the internal crossflow coolant supply induces swirling flow inside film hole, making the interaction between coolant jet and primary flow more complicated. At lower blowing ratio, the laterally averaged adiabatic film cooling effectiveness for the internal crossflow coolant supply is increased a little compared with the plenum coolant supply case when the crossflow ratio is less than 1.0,and the situation is reverse when the crossflow ratio is greater than 2.0. At higher blowing ratio, the internal crossflow coolant supply is benefit to improving film cooling effectiveness, especially when the crossflow ratio equals to 2.0. Under higher crossflow ratio, the lateral spreading of the film coolant increases obviously with increasing of blowing ratio.