Relaxation process of ammoniated NH4 radicals in the first excited state is studied with a femtosecond pump-probe technique and a time-of-flight mass spectroscopy. Small ammoniated NH4 radicals are produced by an ArF-excimer laser photolysis of ammonia clusters. The decay of the excited state is probed by a resonance-enhanced two-photon ionization method. NH4 radical in the first excited state is found to have a lifetime of an order of 50 ps. The lifetimes of NH4(NH3)(n) decrease from 1.4 ps (n = 1) to 300 fs (n = 4) with increasing the number of solvent molecules. The large reduction of the lifetimes is ascribed to the enhancement of nonradiative decay process as a result of the drastic change in the electronic structure of these clusters with the addition of ammonia molecules. For larger clusters (n greater than or equal to 5), the lifetime becomes almost constant at 120 fs. The observed size dependence of the lifetimes is discussed in relation to the formation of a one-center ion-pair state by the spontaneous ionization of NH4 in ammonia clusters.