An electrode used to form a film cooling hole in an air cooled turbine airfoil using the electrode discharge machining process. The electrode includes a number of film hole forming projections each having a metering hole forming section and a diffusion forming section. The diffusion forming section has a trapezoidal shape in which two sidewalls have both an outward expansion and are slanted outward to form a convergent diffusion or slanted inward to form a divergent diffusion.