Alumina (Al2O3) thin films were prepared from aluminum triisopropoxide (Al(OCH2CH3)(3), ATI). A CVD reaction follows two reaction paths; one is a gas-phase reaction and the other is a surface reaction. In the former path, the reaction takes place in the gas phase and the intermediate is rapidly deposited onto the film. In the latter path, the reactant is rapidly diffused and adsorbed onto a film surface, on which the reaction takes place. To clarify which path is major in the alumina CVD reaction, the effect of the ratio of reactor volume to substrate surface area (V/S) on the deposition rate is investigated. The deposition experiments were performed in a hot-wall quartz tube reactor with different inner diameters. The reaction rate is the first order with respect to the ATI concentration and the gas-phase decomposition of ATI is the major reaction path. The reaction rate equation of alumina is determined in the temperature range of 900 to 1,100 degrees C. The activation energy is found to be 179 kJ/mol.