Molecular diffusion and crosslink between sizing, epoxy, and hardener agent were simulated to elucidate forming process of interphase in carbon fiber/epoxy composite, by using a multiscale method, in which fully atomistic molecular dynamics (MD), coarse-grained dissipative particle dynamics (DPD) and Monte Carlo-like polymerizing models are used in combination. It shows that mutual diffusions of the three components tend to result in gradient distributions in the fiber vicinity crossing the interphase transition region. The diffusion behavior is extremely restrained by the crosslink reactions. The results indicate that a period of diffusion before the initiation of chemical reaction is necessarily important to obtain sufficient crosslink within the interphase. The sizing amount and the sizing compositions have significant influence on the interphase region. Thicker sizing layer leads to less crosslink and wider transition region, whereas existence of hardener agent in the sizing layer can generate higher crosslink density without changing the width of the interphase. These results deepen our understanding on molecular formation and optimization of the three-dimensional interphase region in carbon fiber/epoxy composites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40032.