The invention provides a backlight unit, a liquid crystal display apparatus and a method of assembling the backlight unit in which a large number of optical sheets can be simply and surely assembled into a chassis in an appropriate order and direction, and is characterized by a first optical sheet which is a reference sheet and second optical sheets which are rest of the optical sheets. The first optical sheet is provided with first identification parts for specifying a layer of respective second optical sheets to be placed, and each of the second optical sheets is provided with a second identification part at a position corresponding to one of the first identification parts based on the layer position to be placed. The optical sheets are superposed so that each of the first identification parts is pared with the second identification part corresponding to the layer specified by the first identification part.