The effects of isoelectronic In-doping on the structural and optical properties of GaN, GaN/Al0.17Ga0.83N and Al0.05Ga0.95N/Al0.17Ga0.83N quantum wells (QWs) on GaN were investigated. QWs were grown by atmospheric pressure metalorganic vapor-phase epitaxy with either H-2 or N-2 carrier gas. Without In-doping, QWs grown in N-2 carrier gas had much higher crystalline and optical properties than those grown in H, carrier gas. X-ray diffraction and photoluminescence studies have revealed that In-doping improves the crystalline and optical properties of QWs, regardless of the carrier gas species used during growth. These improvements are more remarkable for In-doping into the well layers rather than into the barrier layers.