A root airfoil of a wind turbine blade has a suction surface and a pressure surface formed at the upper portion and at the lower portion of a chord line which connects the leading edge and the trailing edge. The operating Reynolds number is 30,000 to 1,000,000, and the angle of attack of the airfoil is 0 to 21 degrees. The maximum lift coefficient is 0.8 to 1.1, and the minimum drag coefficient is 0.02 to 0.07.