Excited state relaxation process of naphthalocyanatovanadium(IV) oxide (VONc) in solutions and in polymer films was studied by transient absorption measurements. The transient absorption decay of VONc in polymer films is faster than that in toluene solution. The excitation power dependence and weight fraction dependence of the excited state dynamics and absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from singlet excited state. The aggregation of VONc is concluded to be useful to ultrafast all-optical light modulation in the near-infrared region.