A nitride semiconductor light emitting device is provided. A core semiconductor region, a first cladding region, and a second cladding region are mounted on a nonpolar primary surface of a support substrate of GaN which is not the polar plane. The core semiconductor region includes an active layer and a carrier block layer. The first cladding region includes an n-type AlGaN cladding layer and an n-type InAlGaN cladding layer. The n-type InAlGaN cladding layer is provided between the n-type AlGaN cladding layer and the active layer. A misfit dislocation density at an interface is larger than that at an interface. The AlGaN cladding layer is lattice-relaxed with respect to the GaN support substrate and the InAlGaN cladding layer is lattice-relaxed with respect to the AlGaN cladding layer.