The present invention relates to a technology of displaying device, and more particularly to a heat-dissipating device of a backlight module, and which is arranged between a backframe and a waveguide of the backlight module for dissipating heat generated from a light source disposed adjacent to the waveguide, wherein the heat dissipating device includes a master heatsink, and an auxiliary heatsink; and wherein the master heatsink has a sidewall extending from an end thereof to in connection to the waveguide, and the auxiliary heatsink is arranged between the master heatsink and the waveguide. The present invention further provides a backlight module incorporated with such a heat-dissipating device. With the combination of the master heatsink and the auxiliary heatsink, a novel heat-dissipating device is created and which effectively upgrades the heat-dissipating performance under the high power density, prolonging the service life of the light source, and the increases the optical characteristic of the displaying device.