Abstract: The extraction of ruthenium from nitric acid medium by trialkylphosphine oxide (TRPO) was studied under a variety of experimental conditions. The influence factors such as contacting time, nitric acid concentration, temperature, TRPO concentration, etc., on the extraction behavior were systematically investigated. Three-stage cross-current extractions were further conducted to reveal the extraction behavior. Moreover, the stripping of ruthenium from the loaded organic phase by nitric acid was also studied. The results from this study indicate that the thermodynamics and kinetics of the speciation and chemical equilibrium of ruthenium in nitric acid medium are quite interactive and thus resulting in complicated extraction behaviors. Furthermore, the extraction of ruthenium from a series of simulated HLLW was conducted and the results provide additional information for the optimization of TRPO process regarding the decontamination of ruthenium from HLLW.