Experiments and numerical simulations were carried out to analyze the soot aggregation mechanism during benzene pyrolysis. Soot was formed by the pyrolysis of 1 mol% benzene (in 99 mol% nitrogen) in an alumina tube, which was kept at 1573K with variations of residence time (0.03 to 0.5 sec). A new cluster-cluster aggregation model called the Aggregate Mean free Path (AMP) model was developed. This model simulated the cluster-cluster aggregation between soot particles and aggregates using the particles or aggregates mean free paths. The projected images of simulated soot were compared with the electron micrographs of experimental soot with the same magnification. Simulated shape, peripheral fractal dimension and size distributions of aggregates were in good agreement with experimental data.