A random copolymer ((U/A10)(165)) bearing pendent ureido groups and a small amount (10 mol %) of primary amino groups exhibits an upper critical solution temperature (UCST). We prepared a diblock copolymer (PMPC20P(U/A10)(165)) composed of water-soluble poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and (U/A10)(165) blocks via reversible addition-fragmentation chain-transfer radical polymerization with postmodification reaction. The subnumbers are the degrees of polymerization of each block. Although in water PMPC20P(U/A10)(165) dissolves as a unimer above the UCST phase transition temperature (T-p), it forms polymer micelles composed of dehydrated (U/A10)(165) cores and hydrophilic PMPC shells. A nanogel was prepared by cross-linking the pendent primary amines in the micelle core using (hydroxymethyl)phosphonium chloride below T-p. NMR and light-scattering data indicated that the nanogel core shrinks upon dehydration below T-p and swells upon hydration above T-p. The nanogel can encapsulate guest molecules such as hydrophobic fluorescence probes and bovine serum albumin (BSA) below T-p mainly owing to hydrophobic interactions in the core. Encapsulated BSA can be held in the nanogel core below T-p and subsequently released above T-p.