Poly(styrene-b-methacrylic acid) block copolymers were synthesized by hydrolysis of poly(stryene-6-tert-butyl methacrylate) block copolymers. The microphase separated structures of these block copolymers were poly(methacrylic acid) spheres in polystyrene matrices. Upon crosslinking the poly(methacrylic acid) spheres with hexanediisocyanate in the solid film, the poly(methacrylic acid) core/polystyrene shell type polymer microspheres were obtained. The packing structures of the microspheres were hexagonal and face centred cubic in two and three dimensions, respectively.