Soot formation during benzene pyrolysis was investigated experimentally and numerically. In the experiment, soot was formed by pyrolysis of gaseous benzene in an alumina reactor. In the numerical simulation, a new model was constructed by combining the discrete-sectional model and the cluster-cluster aggregation model. The new model describes the rate of benzene pyrolysis, nucleation and the formation, growth and aggreation of primary particles. In the reaction of primary particle growth, fusion of primary particles by sintering in an aggregate is also considered. Calculated results of the size distribution of primary particles and the shape of aggregates are in good agreement with experimental ones. Furthermore, the calculated results indicate that the fusion of primary particles by sintering significantly affects the primary particle diameter and the shape of aggregates.