A heat dissipating device is adapted to be mounted in an electric nail gun that includes a flywheel being rotatable about an axis, a first pulley being spaced apart from the flywheel, and a transmission belt being trained on the first pulley. The heat dissipating device includes a second pulley, a weight disc and a heat dissipating member that are adapted to be connected coaxially and co-rotatably to the flywheel. The second pulley is adapted to be trained by the transmission belt. The heat dissipating member includes a plurality of fan blades extending in a direction of the axis such that, during co-rotation of the flywheel, the weight disc and the heat dissipating member, the fan blades create a flow of air for cooling.