A heat-dissipation device adopted for a safety helmet which includes a heat-transfer unit that functions as a heat pipe, a heat-dissipation unit connecting to the heat-transfer unit, a vent formed in a front of the heat-dissipation unit that can be closed and opened alternatively by a shutter unit in order to adjust the capacity of heat-dissipation thereof, and a covering unit made of insulative materials and spreading over the heat-dissipation unit. Whereby the heat-transfer unit provides a kind of two-phase flow that is capable of conducting heat rapidly, so as to remove heat gathered in the helmet and improve the comfort level for the wearer.