A heat dissipation device for a flashlight includes a lighting portion and a handheld portion. The lighting portion is provided inside an outer housing and includes a lighting source adjacent to a front light cover and a light reflector. An end of the lighting source is connected to a power supply end via power wires, and one side of the handheld portion is attached with an elastic clip. A heat dissipating fin is attached to a back portion of the lighting source inside the outer housing. Air vents are formed on a rear side circumference of the outer housing. Whereby, heated air generated by the lighting source is exhausted rearward from a side of the heat dissipating fin and along the air vents on the rear side circumference of the flashlight to maintain good air circulation between the flashlight and the user's garment.