Provided is a backlight module including a light source, a light guide plate, and a composite color-conversion layer. The light source emits a blue light. The light guide plate is optically coupled to the light source and the blue light transmits through the light guide plate. The composite color-conversion layer is disposed on the light guide plate. The composite color-conversion layer includes at least three different populations of quantum dots. The at least three different populations of quantum dots at least include a plurality of cyan quantum dots or a plurality of yellow quantum dots.