||Poly(N-vinylamine) (PVAm) is a typical addition polymer having a large amount of primary amine groups, but controlled radical polymerization of PVAm have been rarely reported. We synthesized poly(4-hydroxystyrene)–b-poly(N-vinylamine) (PHS-b-PVAm) by Reversible Addition Fragmentation Chain-Transfer (RAFT) polymerization. We use ‘switchable’ RAFT agent to make poly(4-acetoxystyrene)-b-poly(N-vinylformamide) (PAcOS-b-PNVF), a precursor of the desired PHS-b-PVAm. The reactivity of C-S double bond on the switchable RAFT agent could be controlled by protonation and deprotonation of Z group which gives a proper control over the polymerization of both PAcOS and PNVF blocks. Hydrolysis of PAcOS-b-PNVF produces a diblock copolymer of PHS-b-PVAm, which has acidic phonolic moeity on the hydroxystyrene block and basic primary amine on the vinylamine block. The diblock polymer self-assembled in water, forming a spherical nanoparticle.