Metalorganic vapor phase epitaxial (MOVPE) growth of AlN and AlGaN was conducted on sapphire and on SiC substrates. AlN and AlGaN free of residual impurities such as oxygen, carbon and hydrogen and relatively low dislocation density can be successfully grown on c-plane sapphire and on 6H-SiC or 4H-SiC substrate if the substrate temperature during growth is higher than 1,623 [K]. One of the important advantages of the high temperature MOVPE growth is that we can successfully grow extremely low dislocation density AlN and AlGaN using epitaxial lateral overgrowth (ELO) technique. We can grow AlN and AlGaN with a dislocation density as low as 107 cm-2 on both substrates. ELO was also conducted for the fabrication of AlGaN-based UV LEDs and LDs. Reduction of threading dislocation density is essential to fabricate high efficiency UV LEDs and LDs. Use of ELO on the reduction of threading dislocation density in AlGaN is very effective to fabricate highly efficient UV LEDs. Details of the ELO of AlN and AlGaN by high temperature MOVPE and performance of UV LED and LD grown by high temperature MOVPE will be discussed.