Phase behaviors induced by solvent annealing in poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN) blend ultrathin films have been investigated by atomic force microscopy and grazing incidence small-angle Xray scattering. Our results indicate that both the phase separation within the blend and the dewetting of the film induced by composition fluctuation take place upon the selective solvent annealing, producing complex structures containing upper droplets (of one phase) and mimic-films (of the other rich-phase). The use of acetic acid (the selective solvent for PMMA) generates PMMA mimic-film and SAN droplets, while the introduction of DMF (exhibiting better solubility for SAN) vapor results in the formation of SAN mimic-film and PMMA droplets. Essentially, the interaction at polymer/substrate interface, resultant wettability of selected component, solubility of PMMA and SAN in adopted solvent dominate not only the phase separation and the dewetting of the whole film but also the synergism of them. (C) 2014 Wiley Periodicals, Inc.