Dynamics of dewetting and phase separation in ultrathin films (thickness is ca. one radius of gyration, ~ 1 Rg) of poly(methyl methacrylate) (PMMA) and poly(styreneran-acrylonitrile) (SAN) blends on Si substrate has been studied by in situ atomic force microscopy (AFM). In the miscible region, a ‘‘spinodal-like’’ dewetting driven by a composition fluctuation recently predicted by Wensink and Je´roˆme (Langmuir 2002, 18, 413) occurs. In the two-phase region, the dewetting of the whole film is followed by phase separation in the droplets, coupling with the wetting of the substrate by the PMMA extracted by the strong attractive interaction between them.