Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate Coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate n(p) is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, n(p) is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity. OH mole fraction, and C2H2 mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence oil soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method. (C) 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.