The very early stage of amorphous SiO2 thermal CVD from tetraethoxysilane (TEOS) oxidized by ozone were observed by atomic force microscopy (AFM) and X -ray photoelectron spectroscopy (XPS). Mono crystalline substrates of Al2O3, Sir and CaF2 were used for examining the dependence of growth mechanism on substrate. The thinnest film deposited on Al2O3 substrate was 1.6 nm thick, and was evaluated to already be a continuous film by XPS. Surface unevenness of Ra = 0.48 nm on this was observed by AFM. This unevenness increased until the SiO2 film grew to be 4.2 nm thick, then decreased to be smooth within the resolution of AFM. The same phenomena were observed on Si and Al2O3 substrates. The film thickness at which smoothing stared were different depending on the substrates-4.2 nm for Si and 3.6 nm for CaF2, respectively. A simulation shows that the appearance of-unevenness is explained by cone structure resulting from growth-rate non-uniformity, and that smoothing is attributed to the effect of surface flow during deposition.