A study on the oxidation of SiO2 sensing layer was done at 950, 1000, 105$0^{circ}C$ under dry O2 atmosphere. The rate determining step around the oxide layer thickness, 1000$AA$ was different with the oxidation temperature, as follows ; ⅰ) linear growth at 95$0^{circ}C$ and ⅱ) parabolic growth at 100$0^{circ}C$ and 105$0^{circ}C$. The flatness of SiO2 film was observed within $pm$1% and surface state charge density was reduced by annealing in N2 atmosphere. Finally, pH sensitivity of SiO2 film, in the range of pH 3-9, was 20mV/pH.