A helmet includes a helmet body, and a cooling fan device mounted in the helmet body to introduce an ambient air into the helmet body. The cooling fan device includes a support base, a cooling fan, a battery, and a control switch. Thus, the cooling fan is operated to introduce the ambient air through the vent hole of the helmet body into the helmet body to provide ventilating and radiating effects to a user wearing the helmet body so as to provide a comfortable sensation to the user.