The self-association behavior of a thermosensitive amphiphilic block copolymer, poly(N-isopropylacrylamide)-b-poly(N-vinyl-2-pyrrolidone) (PNIPAM-b-PNVP) in water upon heating was investigated by static and dynamic light scattering (SLS and DLS), small-angle X-ray scattering (SAXS), and pulsed field gradient NMR (PFG-NMR). Combining SLS and DLS with SAXS and PFG-NMR, we conclude that above 40 degrees C, the unimer or a few arms star micelle of the copolymer coexists with large spherical particles of uniform density, which should be regarded as concentrated-phase droplets produced by a liquid liquid phase separation. The spherical particles of PNIPAM-b-PNVP samples with the PNIPAM content less than 0.32 became smaller with increasing temperature. This temperature dependence was explained in terms of the packing parameter for amphiphiles.