GPC elusion phenomenon of a PS-PMMA diblock copolymer is not simple because of heterogeneous nature of the component polymers. A block copolymer sample which has larger molecular weight does not always elute faster (have larger hydrodynamic volume 111 solution) shall a lower molecular weight one. Addition of a non-solvent gives size contraction of five different PS-PMMA sample diblock copolymers and the size contraction is detected by GPC elution volume. Addition of MeOH which is selective to PS component results effective contraction of PS-rich block copolymers. And addition of n-hexane results the opposite effect. This size contraction of sample depends on the relative ratio of the molecular weight of component polymers (mole percent of PS).