A backlight module includes a lighting board, a light guide panel, a shielding sheet, and a protrusion structure. The lighting board includes two non-intersected traces, a plurality of microstructure regions and at least one illuminant unit. The illuminant unit is connected between the microstructure regions. The microstructure regions do not overlap with the two non-intersected traces. The light guide panel is disposed above the lighting board, and the shielding sheet is disposed above the light guide panel. The protrusion structure is formed on the shielding sheet, protrudes towards the lighting board, and is between the microstructure regions. The protrusion structure diffuses light of the illuminant unit at an early section of its light path to enter into the light guide panel for lateral transmission. The plural microstructure regions reflect and recycle the light at a following section of the light path to jointly enhance the uniformity of illuminating a keycap.