We studied the formation of non-aggregated Li4Ti5O12 particles by reaction between lithium nitrate and titanium tetraisopropoxide in a methane/oxygen coflow diffusion-flame reactor. We obtained the powder consisting of phase-pure Li4Ti5O12 particles after calcination at 1,073 K. The particles produced in the flame reaction from the spray solution contained an excess amount of lithium corresponding to 25% of the stoichiometric amount. A change in the proportion of virtually non-aggregated particles in TEM images was observed using a rapid cooling of the entire flame aerosol by blowing with cold quenching gas of Ar followed by supercooling in a Laval nozzle placed above the flame. The proportion of non-aggregates was 28% for the particles produced without any cooling steps. When the flame aerosol was supercooled in the Laval nozzle after blowing with 203 K Ar quenching gas, the proportion of non-aggregates was as large as 88%. Thus, rapid cooling in the region of the flame tip is quite effective for preventing particle aggregation.