We follow the time development of the micro-domain structure in symmetric polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer thin films during acetone vapor treatment. Besides the highly ordered nanoscopic spheres or stripes as reported previously, a novel so-called flower-like pattern, which comprises six PS spheres and each PS sphere belongs to three "flowers" is formed. This finding is very helpful to discuss the highly ordered nanoscopic sphere formation process.