Hydrogen donor visbreaking and conventional visbreaking processes of vacuum residue of Venezuelan synthetic crude oil were investigated in an autoclave. Phase changes of the two reaction systems were observed by a microscope during the thermal conversion. The stabilities of two kinds of visbroken oil were characterized and their SARA group compositions were analyzed. The results show that amorphous micro-particles' formation, growth and aggregation can be observed clearly by the microscope with the increasing of time at 425°C. Resins and asphaltenes contents in two visbroken oils are both decrease. Compared to conventional visbreaking process, hydrogen donor in hydrogen donor visbreaking process can inhibit the growth of amorphous micro-particles to postpone the phase separation. Moreover, hydrogen donor can inhibit the coke formation and improve the stability of visbroken oil.