Star polymers bearing vinyl-functionalized hyperbranched cores were successfully prepared at moderate-to-high monomer conversions (ca. 80%) via a modified "arm-first" strategy. This was achieved by reversible addition -fragmentation chain transfer polymerization of a methacrylate type asymmetrical divinyl monomer with thiobenzoate-capped polystyrenes as macro-chain-transfer agents (Macro-CTAs). Neither intermolecular nor intramolecular cross-linking occurred up to high levels of monomer conversion. The molecular weights (MWs) of Macro-CTAs significantly influence the structures of the resultant star polymers. A lower-MW Macro-CTA favors the formation of a star copolymer bearing big core and multiple short arms, whereas a higher-MW one is confined to the architecture bearing small core with only a few long arms. Moreover, the ca. 70% pendant vinyl groups remained in the hyperbranched core can be directly modified using reactions such as epoxidation and UV initiated thiol-ene addition reactions, resulting in star polymers with various functional groups in the core.