The largest collision kernel is between a small particle with high mobility, and a larger particle which presents a large collision cross-section. However, most coalescence models are limited to the analysis of equivalent particle sizes. In this paper we focus on understanding the coalescence mechanisms of unequal sized nanoparticles. We have studied the coalescence of pairs of silicon nanoparticles of volume ratios between 0.053 and 1 with 10000 (at 1500 K) and 1600 (at 1000 K) silicon atoms using, molecular dynamics (MD) simulation under constant temperature conditions. We found that the convection processes, and deformation of the smaller particle dominated the coalescence process for liquid-like particles. On the other hand, for near solid-like particles diffusion processes dominated the coalescence of nanoparticles. Coalescence processes become faster when the ratio of two particle sizes (smaller/larger) approaches zero. Most importantly we find that the Koch-Friedlander (KF) accurately predicts the coalescence time of two unequal sized particles when benchmarked against the MID simulation results, and that the characteristic coalescence times is independent of the volume ratio of the coalescing partners. (C) 2005 Elsevier Ltd. All rights reserved.