Poly[styrene(S)b-2-vinyl pyridine (2VP)] diblock copolymer (SV1, M(n)BAR = 2.1 X 10(4), 2VP = 14.0 mol%) and poly[S-b-isoprene (IP)-b-2VP] triblock copolymer (SV2, M(n)BAR = 1.1 x 10(4), 2VP = 12.4 mol%, IP = 3.6 mol%) were synthesized by anionic addition polymerization. By blending SV1 and SV2 from benzene solution at 2 wt% polymer concentration, the co-micelled microphase separated film with P2VP spheres in a PS matrix was obtained. Monodispersed P2VP core-PS shell type polymer microspheres which had two types of PS arms owing to the PS sequences of SV1 and SV2 were synthesized by crosslinking the co-micellized films with 1,4-diiodobutane. Arm-numbers of the microsphere were between 440 and 540. PS arms owing to SV2 in the microsphere were cut off quantitatively from the microsphere by degradation of the PIP blocks of SV2 on the surface of the P2VP core with ozone. By changing the volume fraction of SV2 in the microsphere from 20 to 40 wt%, the arm-number of the microsphere could be controlled quantitatively in the range of 88-540.