A backlight module (30) includes a light source (42) for emitting light, a light guide plate (50), and a reflective cover (41). The light guide plate includes a light incidence surface (51), a light-emitting surface (53) connecting with the light incidence surface, and a bottom surface (52) opposite to the light-emitting surface. The light source faces the light incidence surface. The reflective cover includes a reflective unit having two reflective surfaces (411, 412) defining a space with a bottom. The light source is accommodated in the bottom of the space. A cross-sectional profile of the reflective surfaces is defined by an equation in a polar coordinate .rho.(.phi.). The equation is:.intg..pi..phi..theta..times..rho..function..phi..times..function..phi- ..theta..times.d.phi..theta..intg..theta..times..function..theta..times.d.- theta..rho..function..phi..rho..times..function..theta..function..phi..the- ta. ##EQU00001## wherein, .rho. represents a polar axis of the polar coordinate, .phi. represents a clockwise angle of the polar coordinate, and .theta. represents an anti-clockwise divergence angle of the light.