Disclosed is a fastening device for a LED light string of a backlight module, including a LED light string and a frame, the LED light string being configured with a substrate and a plurality of LEDs mounted onto the substrate, wherein at least a retainer is incorporated on the substrate, and the frame being provided with a notch matchable with the retainer such that the LED light string can be securely attached to the frame. The retainer can be made individually and a then attached to the backside of the LED light string. In addition, with the retainer readily snapped into the notch of the frame, a distance between to the LED light emitting surface to the frame is also reduced, and this helps to realize the design of narrow-boarder of the liquid crystal display.