The composition and the evolution of the 355nm laser ablated plasma from a La~2O~3 target both in vacuum and in ambient O~2 were investigated by using time-resolved emission spectrometry. Our results show that the excited La^+^* and LaO^* are two main emission species. It is suggested that there are two pathways for the formation of LaO^* in ambient O~2, one is the direct production via laser-target interaction, and the other is the formation via the oxidation of La and La^+. The dependence of the delay time and the intensity of the emission on the laser fluence, the observation distance away from the target surface, and the ambient O~2 pressure were examined. A kinetic model based on the interactions between the ablated species and the ambient O~2 is proposed to explain the experimental results satisfactorily. The mechanism of the plasma formation and evolution is also discussed.

Key words: PLASMAS, LUMINESCENT SPECTRA, LANTHANUM OXIDE