To clarify the mechanism of fine-particle formation processes in a multilamination type microreactor that proceeds mainly due to molecular-diffusion-induced mixing, the relation between the size and distribution, and reaction-operation condition in experimental and numerical approaches is investigated. For experimental study, a concentric microreactor was used, and titania fine-particle formation was examined as a model reaction. Based on the hypothesis on the fine-particle formation mechanism, from the experimental results derived, a relatively simple calculation model was made, and validity demonstrated by showing the coincidence of the calculation results and the experimental results. Also, the calculation model was used to establish a technique for predicting the reactant concentration profiles in the microchannel, particle-growth rate, number of formed particles, and nucleation region. Thus, the proposed analysis method will be useful as a guideline for designing microreactors to form fine particles, and will provide industrially information. (c) 2006 American Institute of Chemical Engineers.