We propose here a new mechanism of droplet coarsening in phase-separating fluid mixtures. In contrast to the conventional understanding that there are no interactions between droplets in the late stage of spinodal decomposition, we demonstrate the existence of interactions between droplets that is caused by the coupling between diffuse concentration change around droplets. We show the possibility that this mechanism plays an important role in droplet phase separation together with Brownian-coagulation mechanism. We also discuss the coupling between hydrodynamic and diffusion modes, namely, ''collision-induced collision'' phenomena. (C) 1997 American Institute of Physics.