A polymer alloy of polyphenylene sulfide resin and a polyester resin with polyethylene terephthalate as a main component, and forming a co-continuous structure with a wavelength of concentration fluctuation of 0.001 to 1 .mu.m or a dispersed structure with a distance between particles of to 1 .mu.m. The co-continuous structure or dispersed structure of the polymer alloy is formed by the phase separation caused by spinodal decomposition. The polymer alloy is miscible when the shear rate is kept in a range from 100 to 10000 sec.sup.-1, and is separated into phases under no shear flow. The polymer alloy of this invention can provide a molded article, film, fibers and the like respectively with excellent mechanical properties at high productivity.