A freewheel structure includes a wheel frame and at least one weight block. The wheel frame has a central axle portion and a flange portion around an outer edge thereof. The axle portion is for insertion of an axle member. The weight block is selectively located between the axle portion and the flange portion, so that the weight block has a certain distance relative to axle portion. The freewheel will generate a greater centrifugal force when it is rotated at a high speed. The inertial effect of the present invention can be enhanced when the rotational speed is increasing progressively, so the curve of inertial weight is elongated obviously. The whole weight of the freewheel is reduced effectively.