The growth of AlGaN films on sapphire substrate by metalorganic chemical vapor deposition are investiga-ted. Through studying the traces of in situ optical reflectivity,we have observed that the quasi two-dimensional growth of AlGaN occurred followed by three-dimensional growth in the initial stage. Due to low surface mobility ofAl atoms, it will take more time to transfer the two-dimensional growth into three-dimentional growth in the process. Since there are parasitic reaction of TMAl and NH3 , the Al content in solid phase ismuch less than that in vapor phase. We have also found that the growth rate ofAlGaN decrease with increasing the flux of TMAl, thismay be attribute to the site-blocking effect in which the competitive adsorp tion of TMAl decreases the TMGa reaction with NH3 on the surface.The possible method of improving the AlGaN quality is discussed.