The phase behavior of a block copolymer and supercritical fluid system was investigated. When a particular block was selectively swollen by the supercritical fluid, the apparent volume fraction of one domain was controlled by the pressure of the supercritical fluid. The morphological variation of polystyrene-b-poly[2-(perfluorooctylethyl) methacrylate]s (PSPFMAs) with different ratios of PS to PFMA and the total degree of polymerization were analyzed. Time-resolved small-angle neutron scattering measurement revealed that lamellar and hexagonal phases coexist metastably, which may be induced by fluctuation of the supercritical fluid.