This paper investigates internal heat transfer of film-cooled leading edge with mainstream flow. The semi-cylindrical leading edge model receives coolant through impinging jets located at the neighborhood rectangular channel. The leading edge has three rows of cylindrical film cooling holes: row 1 located along the stagnation line (0 degrees) and rows 2 and 3 at +/- 40 degrees measured from the stagnation line. All film cooling holes are at an inclined angle of 25 degrees relatives to the surface. There are two impinging jet designs in this study: the normal jet and the tangential jet. The normal jet has one row of normal jet impinging holes. After jets impinging on the inner wall of the semi-cylinder stagnation line, coolant spreads out through film cooling holes. The tangential jet has two rows of tangential jet impinging holes. Swirl flow is generated when jets enter the semi-cylinder from two sides of the semi-cylinder. Mainstream Reynolds number is about 100,000 based on the outside diameter of the leading edge cylinder, and the mainstream turbulence intensity is about 7%. Leading edge detailed internal heat transfer distributions are measured by using transient liquid crystal method. Three different coolant jet Reynolds numbers are tested (Re-j = 5000, 10,000 and 15,000), corresponding to averaged blowing ratios about 0.77, 1.54, and 2.31. The experimental results provide useful information for the jet impingement cooling design, especially the leading edge region is under the conditions of mainstream flow and external film extraction. CFD simulations are performed to present the velocity field and compare the heat transfer results with experimental data. (C) 2019 Elsevier Ltd. All rights reserved.