Narrowly size-distributed thermosensitive poly(N-isopropylacrylamide) (PNIPAM) nanocapsules with a hydrophilic core composed of an aqueous solution of cobalt tetrafluoroborate (CoTFB) were synthesized via a one-pot polymerization of N-isopropylacrylamide (NIPAM) and the cross-linking agent N,N'-methylene bis(acrylamide) (MBA) in inverse miniemulsion. CoTFB not only worked as template and lipophobe, but also promoted the dissolution of NIPAM in water by the formation of a complex between NIPAM and CoTFB. The formation of PNIPAM nanocapsules was verified by transmission electron microscopy (TEM) and atomic force microscopy (AFM). This is the first report to synthesize nanocapsules in inverse miniemulsion by following a phase separation mechanism as proposed by Tiarks et al. in direct miniemulsion [Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 908-918]. The particle size distribution was narrowed by increasing the weight content of CoTFB, MBA, and water. The particle size of the nanocapsules could be conveniently tuned by varying the weight content of CoTFB and MBA. The as-prepared PNIPAM nanocapsules showed reversible thermosensitive properties. An ordered monolayer of PNIPAM nanocapsules could form by controlling the evaporation of apolar solvent, and moreover the size and thus interparticle distance of PNIPA M nanocapsules in the ordered arrays could be tuned by drying the sample under different temperatures below and above the lower critical solution temperature (LCST) of PNIPAM.