A headlight for bicycle includes an insulative barrel and including peripheral openings; a lens mounted in the barrel; a heat sink including parallel grooves on its outer surface, and water resistant rings mounted thereon; a PCB fitted onto an end of the barrel; a reflector mounted inside the barrel; an LED bulb fitted in the reflector; and a conductor electrically interconnected the bulb and the PCB; and a power supply releasably secured to the barrel and including a switch. The switch is manually movable between on and off position and is adapted to electrically connect the power supply to the bulb when the switch is in the on position, and in response to disposing the switch in the on position, heat generated by the bulb is dissipated through the conductor, the heat sink, and the openings.