The self-assembly of star-block copolymer (ByAx) n in a selective solvent for its outer block is studied by using dissipative particle dynamics, where (ByAx)n denotes a n-armed star with each arm comprising of y solvophobic B-segments and x solvophilic A-segments. The effects of arm number, arm length, block length ratio y/x, solvent quality, and component compatibility on mean aggregation number 〈p〉 are examined. Unusual micellization behaviors are observed. The total number of arms in a multimolecular micelle n* is invariant with the arm number but grows with the segment concentration ψ, which is different from typical micellization of short-chain surfactants. For a given ψ, multimolecular micelles are formed for stars with n<n*(ψ) and the mean aggregation number follows 〈p〉=n*/n. As n≥n*, only unimolecular micelles exist in the solution. According to the n*-ψ relation, the critical multimolecular micelle concentration is found to grow linearly with arm number. As solvent quality deteriorates or component compatibility increases, the total arm number within a micelle rises.