Surface of commercial silica particle was hydrophobically modified with octyltriethoxy-silane (OTES) in an aerosol spray reactor. When the reactor temperature was higher than 250degreesC, one liquid droplet produced one agglomerated particle. However, when the reactor temperature was lower than 250 degreesC, the original droplet shape was completely disappeared and particles were produced in a dispersed state. Based on the Fourier transform-infrared (FT-IR), thermo gravimetric analysis (TGA) and scanning electron microscopy (SEM) results, competition between the evaporation of solvent and reaction of OTES with the silica surface was proposed to be responsible for the morphology dependency on the reactor temperature. Hydrophobicity of the modified particle was quantitatively measured by ethanol volume ratio (EVR) and contact angle. The hydrophobicity was increased as increasing the concentration of OTES and had the optimum point with the reaction temperature. It was found that the discrepancy of the hydrophobicity measured by the EVR and contact angle gave information on the degree of agglomeration of particles produced by the aerosol spray reactor.