A blade mounting structure of a lawn mower includes: a drive hub mounted on an output shaft of a power source and protruding radially outward beyond an outer periphery of the output shaft; a friction member for transmitting driving force from the drive hub to a blade holder, the drive hub and the friction member having respective curved surfaces slidable on each other when the drive hub and the friction member are in abutting engagement with each other; a grass cutting blade attached to the blade holder; and an energy absorbing member disposed in a space defined by the blade holder and members peripheral to the blade holder, the energy absorbing member being constructed to maintain its shape during normal grass cutting condition but deform when a load exceeding a predetermined value acts on the energy absorbing member.