Control of defect and strain in AlGaN layer and the performance of the UV devices were discussed. It was shown that the crystalline quality is significantly improved when an AlGaN layer is grown on a high-quality GaN layer. It was found that a combination of low temperature interlayer and heteroepitaxial lateral overgrowth are effective to obtain crack-free and low dislocation density AlGaN, and thus achieving high-performance UV light emitting diodes (LED). Even after fabricating on the low dislocation density, performance of the UV LED was found to be far behind that of visible nitride-LED.