Smooth, uniform and transparent aluminum oxide films were deposited at low temperature (less than or equal to 500 degrees C) by r.f. plasma enhanced chemical vapor deposition (PECVD) with AlCl3, H-2 and CO2 as the reactants. The measurements of mechanical and optical properties demonstrate a potential application of PECVD aluminum oxide film as an optical protective coating, The variation of film hardness and density with deposition parameters is appal ent and shows a similar dependence while the composition stoichiometry is not so influenced. For amorphous thin films incorporating hydrogen and/or microvoid with no apparent variation on composition stoichiometry, the film hardness is dominated by density and a function near linear can be constructed as energy density is taken into account as an apparent character of hardness.