The particle size distribution of carbon nanoparticle produced from benzene with acetylene and methane was measured, and the effect of the polycyclic aromatic hydrocarbons (PAHs) on carbon nanoparticle formation was investigated. Carbon nanoparticle was generated in an alumina reaction tube heated by an electric furnace by feeding the feedstock and diluting nitrogen into the furnace. The residence time was varied by total flow rate. When the maximum temperature in the reaction tube was 1515±15 K, the carbon nanoparticle concentration was highest when acetylene was added, suggesting that the addition of acetylene promoted the formation of carbon nanoparticle. On the other hand, when methane was added, the approximate shape of the particle size distribution was almost equal to that of the benzene-only feedstock when the residence time was short, and almost equal to that of the acetylene-added feedstock when the residence time was long. Therefore, the growth of PAHs influences carbon nanoparticle formation when residence time is short, and the carbon content of the feedstock has a greater influence on carbon nanoparticle formation than the growth of PAHs when residence time is long. At a maximum temperature of 1300±10 K, the addition of methane decreased the number concentration.