The use of nano alumina thin film by hybrid sol-gel route to protect steel from high-temperature oxidation is limited due to the low thermal stability of this coat. In this work, study has been conducted to minimize this effect. The chemical compositions of solid gel (xerogel) obtained from normal alumina hybrid sol-gel were thermally varied, and then recombining with water to form the modified coating solutions at constant concentration of 0.25mol/l alumina. Laser diffraction technique has been used to measure the growth of clusters sizes in different modified solutions. X-ray diffraction analysis to characterize the alumina phase's changes as a function of temperatures. Also gravimetric weight change method to determine the oxidation rate of steel before and after coating. The results showed five changes in alumina phase during heating to 9500C, which are similar to that recorded for the natural ore of boehmite, and the partial removal of non stable by-products constituents associated with xerogel has considerable effect on the degree of particles aggregation and agglomeration in solutions. This worked to improve the thermal stability of the coating layer, and show considerable resistance to cracks formation. At 40-50% removal, the oxidation rate of HSLA steel specimens was enhanced by 88 % at 9500C compared to the steel before coating.