The solvent-induced spherical structure in a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) block copolymer was obtained and stabilized by preparing both the bulk and thin films from propylene glycol methyl ether acetate (PGMEA) solutions. The diblock copolymer possessed a total molecular weight of 42 kDa with a PS volume fraction of 72.2%, and it formed a cylindrical phase structure in the equilibrium bulk state. During thermal annealing, only changes in the sphere size and packing rearrangement were found. In contrast, a unique structure evolution route was observed during solvent treatments. Under a controlled vapour of a PS selective solvent, an oscillation of the structural transition between spheres and cylinders was observed in the thin films. The kinetics of this oscillation of structural transition was found to be closely related to the solvent vapour concentration and film thickness. This experiment revealed a unique ordering pathway towards the equilibrium structure in the thin film for this strongly segregated PS-b-PDMS diblock copolymer.