This paper presents an experimental investigation of the rectangular diffusion hole. The effects of rectangular aspect ratio and lateral diffusion angle on film cooling effectiveness were studied at a low-speed flat plate experimental facility. The pressure sensitive paint measurement technique was employed to determine the adiabatic effectiveness. The experiments were performed at a density ratio of DR=1.38 and a mainstream turbulence intensity of Tu=3.5%. The blowing ratio was varied from M=0.5 to M=2.5. Three aspect ratios and three lateral diffusion angles were chosen to match the semi-circle and straight line sidewall shape of the rectangular cross section. A comparative investigation was performed among a typical fan-shaped hole and ten rectangular diffusion holes. The experimental results exhibited diversified film distribution patterns of the rectangular diffusion hole, including single-, bi-, and tri-peak patterns. The overall cooling effectiveness increased with the increase of rectangular aspect ratio. The improved magnitude was amplified as blowing ratio increased. The holes with semi-circle sidewall were shown to be more suitable for high blowing ratio conditions. The maximum increase of cooling effectiveness was over 70% compared to the fan-shaped hole. The reduction of the lateral diffusion angle affected the film distribution pattern significantly, thereby influenced the cooling effectiveness. To obtain a fixed coverage ratio of film hole row, the rectangular diffusion hole with a larger cross-sectional aspect ratio and a slightly smaller lateral diffusion angle is a preferred scheme.