A detailed kinetic mechanism of aromatic growth and particulate formation is presented, and it is tested over a range of different operating conditions in rich premixed laminar flames of ethylene. The model includes reaction pathways leading to the formation of molecular particles and their coagulation to soot by using a discrete-sectional approach for the gas-to-particle process. Good predictions are obtained of major oxidation and pyrolysis products, as well as of trace species, particulate concentrations, and particle size distributions. At low flame heights and in nonsooting conditions, the model predicts particle size distribution functions with a single mode centered at about 2 nm, in good agreement with experimental data. At increasing flame heights in sooting flames, the particle size distribution function develops toward a bimodal shape. Modeled data shows the formation of a large number concentration of particle smaller than 3 nm, which are not currently detected by standard techniques based on the measurement of particle mobility. Sensitivity of the predictions to the particulate-phase reaction rates is performed. Model analysis shows the importance of the mechanism of aromatic-molecule addition to aromatic radicals in the formation of nanosized particle and the importance of acetylene addition in soot loading.