A back light module includes a frame body, a back board, a reflecting sheet, a light-guiding plate, a lighting member, an optical film and an upper cover. The frame body and the back board are formed integral. The frame body has its outer peripheral edge formed with lots of radiating fins, and the back board and the radiating fins are respectively disposed with numerous radiating grains for increasing the heat dissipation area and elevating heat dissipation effect of the back light module, able to effectively lower the working temperature and prolong the service life of the lighting member, and elevate the working efficacy of the lighting member and economize cost.