The effect of primary particle size of the raw alumina powder on the microstructure and light transmittance of the 1 μm thick α-alumina film was investigated. Granules made from four kinds of alumina powders with different primary particle sizes were sprayed to deposit the alumina films on glass substrates at room temperature in a vacuum. Grains of the alumina film were mostly not much bigger than 10 nm for all the samples. Both the film surface roughness and the interface roughness between the alumina film and glass substrate of the samples increased as the primary particle sizes of the alumina powders increased. A small number of hair-like flaws were observed in the alumina films prepared from the coarse powders. The variation of light transmittance was explained in part by change of surface roughness, the interface roughness, and flaw size of the alumina film according to the alumina primary particle size.