Thermoresponsive polymer-encapsulated silica hybrid nanoparticles were fabricated via self-assembling of block copolymer in aqueous solution into micelles and subsequent sol-gel process inside the micellar core. Poly(N-isopropylacrylamide)-b-poly(gamma-methacryloxypropyltrimethoxysil ane)(PNIPAM-b-PMPS) was prepared by successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and gamma-methacryloxypropyltrimethoxysilane (MPS) in 1,4-dioxane. In aqueous solution, amphiphilic PNIPAM-b-PMPS self-assembles into micelles with PMPS core and PNIPAM shell. Base-catalyzed sol-gel process inside PMPS core results in PNIPAM-encapsulated silica hybrid core-shell nanoparticles. Transmission electron microscopy (TEM), dynamic light scattering (DLS), and static light scattering (SLS) studies reveal monodisperse hybrid nanoparticles with densely grafted PNIPAM brush at the surface of silica core. Grafted PNIPAM brush shows thermoresponsive two-stage collapse upon heating. Because of the high conversion of RAFT polymerization of MPS, we also show that a one-pot synthesis of thermoresponsive hybrid nanoparticles is feasible. This is the first report of stimuli-responsive hybrid core-shell nanoparticles via the block copolymer self-assembling approach.