Liquid-liquid equilibrium data of quaternary system including phosphoric acid, nitric acid, water and tri-n-butyl phosphate (TBP) was determined at 303.2 K and atmosphere pressure. It obtained the distribution coefficients of phosphoric acid and nitric acid, separation factor of nitric acid on the base of phosphoric acid. It showed that the distribution coefficients of phosphoric acid increased with the increase of phosphoric acid and nitric acid contents. Two association models of Othmer-Tobias equation and Bachman equation were used to test the reliability of the experimental data, and the theoretical model NRTL was used to correlate the liquid-liquid equilibrium data. The correlation coefficient showed a fitting result and that the achievement was reliability. In addition, the type of the extracted complex between phosphoric acid molecule and TBP molecule in nitric acid solution is analyzed as (1.2-1.4) H3PO4 center dot 1.1TBP. (C) 2015 Elsevier B.V. All rights reserved.