A stepped light guide has an incident surface, an emitting surface and a stepped reflecting structure. The emitting surface is adjacent to the incident surface. The stepped reflecting structure is adjacent to the incident surface, faces the emitting surface and has a proximal end, a distal end, multiple planar surfaces and multiple inclined surfaces. The proximal end is adjacent to the incident surface. The distal end is opposite to the proximal end. The planar surfaces are parallel to and separated from the emitting surface by a depth, the depth being gradually reduced from the proximal end to the distal end. The inclined surfaces are formed between every two adjacent planar surfaces. The light guide effectively changes a linear or point light source into a surface light source having uniform illumination.