A fan blade structure of a fan such as, for example, a mixed-flow cooling fan for an automotive internal combustion engine, comprising a blade member having an annular boss portion having a cylindrical outer peripheral surface and a plurality of blade portions integral with and radially projecting outwardly from the boss portion, and a collar member secured to the blade member and having a frusto-conical outer wall portion surrounding the boss portion of the blade member and tapering toward the front end of the boss portion and an inner flange portion circumferentially extending along the boss portion of the blade member and attached to the rear end face of the boss portion, the outer wall portion of the collar member being formed with slots angled and configured conformingly to radially inner end portions of the blade portions, respectively, of the blade member and open at the radially reduced axial end of the collar member.