A fixing apparatus, applied to a diffuser plate of a backlight unit (BLU), comprises a receiving portion and a positioning portion. The receiving portion is symmetrically formed at a first centrosymmetric line of the diffuser plate, a first pair of edges of the receiving portion set at two sides of the first centrosymmetric line. The positioning portion is located within the receiving portion, and a first distance is formed between the positioning portion and the first pair of edges. The first distance is determined according to a mechanical tolerance of the diffuser plate. Movement of the diffuser plate along a direction of a second centrosymmetric line perpendicular to the first centrosymmetric line, is constrained by the positioning portion. The receiving portion and a positioning portion may comprise a fixing hole and a positioning pin. Alternatively, the receiving portion and the positioning portion comprise a fixing groove and a positioning piece.