Abstract The new phase behavior prediction method of petroleum fluids, which was recently developed by Manafi et al. for simple petroleum fluids, has been modified and extended for application to complex petroleum fluids. A combination of the Peng–Robinson equation of state for phase behavior prediction and the Riazi and Mansoori equation of state for density prediction has been applied. The proposed method is applicable for various equations of state as long as they give accurate prediction of phase behavior and densities of hydrocarbons and their mixtures. The composition of petroleum fluids is described by discrete and plus-fraction parts. For the plus-fraction of petroleum fluids, appropriate distribution functions for molecular mass, true boiling point and specific gravity are used. The distribution functions required an average value of molecular mass, and specific gravity of the plus-fraction as input data. Phase behavior calculations are performed for prediction of saturation pressure as well as flash liberation, differential liberation and flash-separation, in a wide range of pressures and temperatures for six different complex petroleum fluids and compared with experimental data. It is demonstrated that the proposed method can predict the phase behavior of complex petroleum fluids mixtures quite accurately.