Abstract: Solvent extraction of Ac from nitric acid solution by tri-alkyl phosphine oxide (TRPO)-kerosene is investigated. The extraction reaction of Ac by TRPO in HNO3 system is studied and the apparent extraction equilibrium constant of Ac is determined. The influence of some parameters on the extraction of Ac is studied. These parameters are: concentration of extractant, acidity of equilibrium aqueous phase, concentration of salting-out agent and concentration of La (Ⅲ). On the basis of experimental data a number of empirical equations are derived, which can be used for the calculation of the distribution ratio of Ac between two phases. The extraction of macro quantity lanthanum by TRPO is also studied. The possibility of separating La and Ac by TRPO extraction is discussed.