A thermosensitive block copolymer, poly(N-isopropylacrylamide)-b-poly(N-viny1-2-pyrrolidone) (PNI-PAM-b-PNVP), self-associates into spherical micelles and/or spherical droplets of the concentrated phase in water upon heating. We have determined the weight fractions of the spherical micelle, the spherical droplet, and the isolated chain in aqueous solutions of three PNIPAM-b-PNVP samples with different contents x of the more hydrophobic PNIPAM in the copolymer, as functions of the temperature by small-angle X-ray scattering (SAXS). When x decreases, PNIPAM-b-PNVP prefers to form the spherical micelle rather than to form the spherical droplet of the concentrated phase. This SAXS result is consistent with the theoretical prediction based on the lattice model.