A newly developed electric furnace was used to pyrolyze benzene, methane, or acetylene diluted by a nonreactive gas, and the carbon aggregates were separated from the hydrogen-rich gases. The temperature was changed up to 2200K for various levels of initial concentration of hydrocarbon and residence time, and how the experimental parameters affected the mean diameter of carbon aggregates was examined. The temperature increase in experiments led to reduction of the mean diameter of the carbon aggregates, whereas increase in the initial concentration of hydrocarbon or the residence time resulted in slightly lifting it. The mean diameter removed of temperature effects correlated well with the initial carbon atom mol concentration of hydrocarbon, N-C, and this parameter was used to develop the experimental equations. This article presents three kinds of experimental equations, and the prediction accuracy was confirmed to be improved when accounting for increasing the experimental parameters.