The coolant flow condition at the film hole entrance has a notable effect on film cooling effectiveness. The previous studies about internal ribbed crossflow effects are the hole with a circular inlet cross section. In this study, a diffusion slot hole with a race-track inlet cross section is examined in a low-speed wind tunnel by the pressure-sensitive paint (PSP) technique. The film cooling effectiveness of three different hole configurations are investigated: a normal diffusion slot hole, a compound angle diffusion slot hole, and a lateral inclination angle diffusion slot hole. Two internal perpendicular crossflow conditions with and without ribs for the 45 deg inline crossflow and the 135 deg counter crossflow are discussed. The blowing ratios are varied from 0.5 to 2.5 at a constant crossflow-to-mainstream velocity ratio of 0.36. A detailed method for the uncertainty analysis of the PSP is also presented. It is found that the normal diffusion slot hole always shows the best cooling performance than other holes. The ribs under the crossflow condition have a positive effect on the film cooling effectiveness for the normal diffusion slot hole even though the compound angle changes the hole’s orientation. However, the ribs have a negative effect on the film cooling effectiveness for the geometrically asymmetric diffusion slot hole.