Abstract: The phase behavior of Liaohe vacuum residue － n-pentane system was investigated with visual observation in a set of Hg-free PVT equipment with two cells and a sight window designated by RUSKA Co. Based on the observation, the p-t phase diagrams with the verging lines of phase transition such as liquid-liquid and liquid-liquid-vapor regions were protracted. The phase characteristics for such system were discussed at 140℃～190℃, 1.0MPa～10.0MPa and the mass ratio of solvent to oil (m_s/m_o) of 3.0~6.0. Under the experimental conditions, the p-t phase diagram is divided into four regions of single liquid, liquid-liquid, liquid-liquid-vapor and liquid-vapor. The phase transition pressures from single liquid to liquid-liquid region are obviously affected by the m_s/m_o ratio; the phase transition pressure increases with the m_s/m_o ratio. However, the m_s/m_o ratio exhibits little effect on the range of liquid-liquid-vapor region. Based on these results, the operating region suitable for the Liaohe vacuum residue － n-pentane deasphalting process was determined.