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.