Anatase titania nanoparticles are prepared using pyrolysis of titanium tetrabutoxide in oxygen free and oxygen containing atmospheres by MOCVD method. Influence of oxygen on the properties of the titania nanoparticles is investigated, followed by discussion. The results show that oxygen influences not only the particle size but also the particle size distribution of the nanoparticles. With increasing oxygen flow rates, the average grain sizes of the nanoparticles decrease and the particle size distributions become uniform. Oxygen exerts great influence on the nucleation rate of the nanoparticles and reaction kinetics occurred in the reactor. The formation of titania nanaoparticles by MOCVD is not a growth controlled process, but is a nucleation rate controlled process.