The visbreaking of heavy oil in supercritical benzene ((SC)benzene) was investigated. By introducing (SC)benzene, the visbreaking originally occurring in the oil phase is transferred into (SC)benzene. The superior diffusivity in (SC)benzene improves the efficiencies of the initiation and propagation of visbreaking network, by which the reaction could be run in the desired tandem structure. By mitigating diffusion limitation to reaction kinetics, the cracking of alkyl substitutes of aromatics vital to viscosity reduction is accelerated. Being the secondary reaction of the cracking, condensation could be terminated promptly at the shortened reaction time necessary for visbreaking. A comparison between the visbreaking in (SC)benzene and supercritical water (SCH2O) confirms the effectiveness of improving diffusion for the optimization on heavy oil visbreaking. Nevertheless, the optimal operating conditions involved must be determined experimentally because of the complicated interaction between phase structure and reaction kinetics. The visbreaking in (SC)benzene shows the advantages of milder operating conditions and better product stability over the visbreaking in SCH2O.