A semiconductor light emitting device including an N-type semiconductor layer, a P-type semiconductor layer, a light emitting layer and a strain relief layer is provided. The light emitting layer is disposed between the N-type semiconductor layer and the P-type semiconductor layer, and the light emitting layer is a multiple quantum well structure. The strain relief layer is disposed between the light emitting layer and the N-type semiconductor layer, and is made of InxGa1-xN, where 0