A backlight module with side fixing frames and assembly methods thereof are provided. The backlight module has a stack assembly including a supporting frame, a reflecting plate and a light guiding plate. At least one optical film is placed on the stack assembly, and at least one lamp assembly is accommodated at each side of the light guiding plate. At least one side fixing frame is disposed at the side of the stack assembly where the lamp assembly is accommodated. The side fixing frame is provided with a groove for containing the lamp assembly and fastening the stack assembly. The side fixing frame fastens the stack assembly and in combination with the optical film forms the backlight assembly. The backlight module is simpler, stronger, and more easily assembled. The manufacturing cost is also reduced.