The present invention provides a valve core of a butterfly valve. The valve core has a lightweight wholly, an improved torque in operation, a dispersed stress in valve closing, and an improved strength. The valve core of a butterfly valve is a circular plate shaped valve core (1) with a boss portion (2) on the top and bottom, and is provided with a cylindrical section hexagon portion (3) in the central area of the boss portion (2), and section circular portions (4) are formed near valve rod embedding portions (8) on the top and bottom outside the section hexagon portion (3). At the same time, the valve wing portions (5) of the boss portion (2) and the valve core are connected via connection R portions (7), the connection R portions (7) are connected via gradually changing connection R portions (6) which become bigger from the top and bottom of the section circular portions (4) to the central portions (3b) of the section hexagon portion (3).