The Al composition ofMOCVD-grown AlGaN alloy layers is found to be greatly influenced by the parasitic reaction between NH3 and TMAl. The growth process ofAlN is carefully investigated bymonitoring the in situ optical reflection. The abnormal dependencies of growth rate on growth temperature, reactor p ressure, and flux ofNH3 are observed, and can be well exp lained by the effect of parasitic reaction. The increase of growth rate with increasing flux of TMAl is found to depend on the growth temperature and reactor pressure due to the presence of parasitic effect. A relatively low growth temperature and a reduced reactor pressure are suggested for the effective decrease of parasitic reac-tion during theMOCVD growth ofAlN and a more effective incorporation of Al into the AlGaN layers.