A back light unit for an outside mirror of a vehicle for optimizing a light-emitting structure of an LED to improve uniformity of light emission and brightness of light so that night visibility for a driver can be improved may include a bottom case, a light source seated on an inner lower side of the bottom case and emitting light when an electric power is supplied, a light guide plate positioned on an inner upper side of the bottom case and uniformly diffusing light emitted from the light source and transferring the light to a prism plate, the prism plate disposed in front of the light guide plate at the inner upper side of the bottom case and refracting and refocusing light from the light guide plate to increase surface brightness, and a top case coupled to a front end of the bottom case.