Femtosecond excited-state dynamics of fullerene-C60 nanoparticles (nC60) having a mean size of 50 nm dispersed in pure water was studied by means of femtosecond transient absorption spectroscopy. The intermolecular charge-transfer (CT) excited state in solid C60 was directly and firstly observed by femtosecond 350 nm and 420 nm excitations, and its intrinsic lifetime of 0.35 ps was found. The CT excited state relaxed to the locally excited S1 state and excimers or directly to the ground state through geminate charge recombination. We also examined the laser fluence dependence of the CT excited-state dynamics. At a high laser fluence, the mutual interactions between neighboring CT excited states were observed immediately after the excitation. The interaction disappeared through the charge recombination in the geminate CT pair or between the neighboring CT excited states with a lifetime of 0.45 ps. After that, the locally excited S1 state decayed with a few ps lifetime independent of the fluence. In this paper, the mechanism and dynamics of the intermolecular CT excited state generated by UV light excitation is discussed in detail.