To study the effect of pretreatment of substrates on the deposition behavior of Al thin films, the surfaces of TiN and SiO2 substrates were exposed to hydrogen plasma or Ar plasma before Al deposition. The Al films were deposited by the pyrolysis of dimethylethylamine alane (DMEAA). A uniform Al film was deposited by the hydrogen plasma exposure to a SiO2 substrate, while island grains were grown by the Ar plasma exposure. The pretreatments of a TiN substrate did not affect the deposition rate of the Al film. The concentration of OH radicals at the SiO2 surface was increased by the hydrogen plasma treatment. We suggest a model in which OH radicals enhance the adsorption of DMEAA on a SiO2 surface, resulting in a larger number of nucleation sites.