Using in situ atomic force microscopy (AFM) and grazing incidence ultrasmall-angle X-ray scattering (GIUSAX), the composition dependence of phase separation, dewetting, and interplay between them in blend [poly(methyl methacrylate) (PMMA) and poly(styrene-ran-acrylonitrile) (SAN)] ultrathin films (similar to 1R(g), radius of gyration) on silicon oxide substrate was investigated. It was found that phase behaviors depended crucially on the composition in blend. First, dewetting morphologies in SAN50 and SAN70 were still under the control of U(q0)/E, which was introduced in our previous work [Polymer 2009, 50, 4456] to distinguish different dewetting pathways. Here, U(q0) and E described the initial amplitude of the surface undulation and original thickness of film, respectively; second, composition produced significant influences on its gradient in composition fluctuation, resulting in the accelerated (or suppressed) dewetting; last, this kind of acceleration or suppression affected the following interplay between phase separation and dewetting so much. Using two samples of "dewetting/wetting-phase separation" and "wetting-dewetting/phase separation" with different components, we discussed the interplay mechanism of them in detail. In conclusion, our results indicate that dewetting, phase separation, and the following interplay between them are sensitive to composition in blend. Furthermore, composition gradient plays an important role in them.