Transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements performed on an InGaN/GaN multiple-quantum-well (MQW) structure deposited on (1 1 (2) over bar 0) and (0 0 0 1) sapphire substrates have been investigated. The grown MQW deposited on (1 1 (2) over bar 0) sapphire substrate are still oriented along the (0 0 0 1) direction due to an unusual growth mechanism, which is confirmed by XRD measurement. TEM measurements of the initial stage of the high-temperature GaN growth after the low-temperature buffer GaN layer on (1 1 (2) over bar 0) sapphire an different from the TEM results obtained for GaN on (0 0 0 1) sapphire substrate, unlike the XRD patterns which were identical. In addition, a further. TEM study indicates that the sample on (1 1 (2) over bar 0) sapphire substrate shows higher threading dislocation density than that on (0 0 0 1) sapphire substrate, a fact that is related to the initial stage of the high-temperature GaN layer. Moreover, the sample deposited on (1 1 (2) over bar 0) sapphire substrate shows a higher density of V-defects incorporating a threading dislocation. The plane-view images of scanning electron microscopy also show that there is a higher density of small dark spots appearing in the paired form compared with that on (0 0 0 1) sapphire substrate, which generally correspond to screw dislocations, It is generally accepted that V-defects and screw dislocations result in phase separation in InGaN around these defects and finally give rise to a strong exciton-localization effect, This result can explain why an enhanced exciton-localization effect was recently observed in the InGaN/GaN MQW deposited on (1 1 (2) over bar 0) sapphire substrate.