The formation of non-aggregated nickel nanoparticles by hydrogen reduction of nickel chloride has been studied in a tubular furnace reactor. A change in the proportion of non-aggregated particles in TEM images was observed using a rapid cooling of the entire aerosol produced in the reactor with jets of N-2 quenching gas and a Laval nozzle placed above the reactor. The proportion of non-aggregated particles was 38% for nickel particles produced without any cooling steps. When the aerosol was supercooled in the Laval nozzle after passing the jets of N-2 quenching gas, the proportion of non-aggregates was as large as 84%. It was found that the rapid cooling in the region downstream of the reactor is quite effective for preventing aggregation of particles.