Micelle formation was studied for a nonamphiphilic diblock copolymer consisting of only solventphilic polymer blocks. Poly(vinyl phenol)-block-polystyrene (PVPh-b-PSt) has no ability to form micelles in 1,4-dioxane, a good solvent for both the PVPh and PSt blocks. The copolymer showed micellization in the presence of 1,4-butanediamine (BDA). It was found that this micellization occurred by hydrogen bond cross-linking between the PVPh blocks in BDA on the basis of H-1 NMR analysis. The size of, unimers (average 6.5 nm) and micelles (average 29.5 nm) prepared from the PVPh-b-PSt with 10K-b-70K as the molecular weight was almost independent of the copolymer concentration, while the aggregation numbers of the micelles increased as a result of increasing copolymer concentration. Dynamic and static light scattering demonstrated that the cmc for the PVPh-b-PSt was determined primarily by BDA concentration at a constant copolymer concentration and by copolymer concentration at a constant BDA concentration.