Two InxGaN1-x/GaN (x=0.15 and 0.18) multiple-quantum-well samples with strained-layer thickness larger/less than the critical thickness, respectively, were investigated by high-resolution electron microscopy, electron holography, and photoluminescence (PL). The PL intensity of the sample with strained-layer thickness larger than the critical thickness was weaker than that of the sample with strained-layer thickness less than the critical thickness by five times. Electron holography revealed that the profiles of the inner potential V0 across the quantum wells GaN/InxGaN1-x/GaN of the samples were not too different. The well feature of the sample with strained-layer thickness larger than the critical thickness was very blurry, especially near the top GaN/InGaN interface. It is suggested that the interface sharpness is most critical for optical property of quantum well devices.