The invention relates to an airfoil for a gas turbine, which comprises a compound fillet disposed between the airfoil and a platform. The compound fillet consists of a first arc and a second arc. A first end of the first arc tangentially adjoins an outer surface of the airfoil. A second end of the first arc tangentially adjoins a first end of the second arc. A second end of the second arc tangentially adjoins a surface of the platform, wherein the following equation is satisfied: 0.15≦R1/s≦0.45, and 0.09≦a/s≦0.27.