A systematic study of the crystallographic and electrical/optical properties of MOVPE-grown InN was performed, and the factors that restrict the quality of MOVPE InN were elucidated. The quality of grown InN is highly dependant on the thermal decomposition of NH 3 as a nitrogen source. At a lower growth temperature (~550 °C) a shortage of active nitrogen, due to a lower decomposition rate of NH 3, causes the formation of N vacancies in the grown InN. With increasing growth temperature, a more stoichiometric crystal is grown and the electrical/optical properties improve. At temperatures above 600 °C, however, deterioration occurs at the N-face of In-polar InN near the substrate interface. This deterioration results in the formation of a porous layer during high temperature (~650 °C) growth. There are a few evidences that show that the hydrogen produced by NH 3 decomposition causes this degradation. Thus, improving the quality of MOVPE-grown InN by changing the growth temperature can be difficult. However, a short growth time at a high growth rate and a relatively high temperature is one effective way to solve this dilemma, and one can achieve carrier concentrations as low as 4×10 18 cm −3 by growth at 650 °C for 30 min.