A backlight module and a liquid crystal display device are disclosed, and the backlight module which comprises a backboard defining a first sidewall with as first engaging device formed thereon. A plastic frame includes a second sidewall with a second engaging device. An elastic member arranged between the first sidewall of the backboard, and a second sidewall or the plastic frame; and wherein the first engaging device interengages with the second engaging device, and the elastic member deformed when the first and second engaging devices interlocked. By this arrangement, the interengagement between the plastic frame and the backboard become more durable and reliable.