A light-emitting device comprises a first type semiconductor layer, a multi-quantum well structure on the first type semiconductor layer, and a second type semiconductor layer on the multi-quantum well structure, wherein the multi-quantum well structure comprises a first portion near the first type semiconductor layer, a second portion near the second type semiconductor layer, and a strain releasing layer between the first portion and the second portion and comprising a first layer including Indium, a second layer including Aluminum on the first layer, and a third layer including Indium on the second layer, wherein the Indium concentration of the third layer is higher than that of the first layer.