The present invention provides a back light module, having an optical element, a light source and an outer supporting means. The optical element includes main body and protrusions which protrude from an edge of the main body. Contiguous protrusions with the main body form a positioning groove. The conjunction region between protrusion and main body is a fillet. The outer supporting means fixes the optical elements and the light source, which has at least one positioning component disposed on the edge of the back plate. The positioning component includes one inner wall near to edge of the main body, a side wall near to the protrusion and a transition wall connecting the side wall and the inner wall. A separate distance g between the fillet and the side wall so that the optical element can be prevented from cracking during the impact or shock test of the back light module.