In modern gas turbines, film cooling is the most common and efficient way to provide thermal protection for hot components. Secondary holes to a primary film cooling hole are used to improve film cooling performance by creating anti-kidney vortices, a technique that has been well documented using flat plate models. This study aims to evaluate the effects of secondary holes on film cooling effectiveness over an airfoil. The film cooling performance and flow fields of a row of primary holes with secondary holes on the pressure side and suction side of a C3X vane are numerically investigated and compared with the results of a single row of cylindrical holes and two rows of staggered cylindrical holes. Cases with different blowing ratios are analyzed. It is shown from the simulation that film cooling effectiveness of primary holes with secondary holes is much better than with a single row of cylindrical holes, and slightly better than with two rows of staggered holes on both pressure side and suction side, with the same amount of coolant usage and blowing ratio. The enhancement is higher on the pressure side than on the suction side. The results show that adding secondary holes can enhance film cooling effectiveness by creating anti-kidney vortices, which will weaken jet lift-off from the primary holes caused by the kidney vortex pair, especially at higher blowing ratios. In addition, film coverage of primary holes with secondary holes is wider and persists further downstream than for a single row of cylindrical holes.