A fixing mechanism for a backlight module and a backlight module are provided in the present invention. The fixing mechanism comprises a plurality of retaining posts and a plurality of retaining sheets. The retaining posts are protruded from a back plate. The retaining sheet is engaged with an upper end of the retaining post for fixing an optical assembly. Moreover, the part of the fixing structure is made of elastic materials, so that it besides can steadily fix the optical film and make no clearance to be created, and also can prevent the optical film from escaping from orientation under the effect of hot-expansion and cold-shrinkage.