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The performance of introducing a free radical initiator into dealkylation-based visbreaking of heavy oil in the N 2 or supercritical benzene (SC benzene) medium was investigated. Calculations based on density functional theory show that the introduction of di-tert-butyl peroxide (DTBP) into heavy oil visbreaking results in the formation of alkyl C radicals similar to those naturally formed in high temperature initiation. The visbreaking in SC benzene environment is sensitive to the introduction of DTBP, which is different from the visbreaking in N 2 environment. According to reaction kinetics analysis, the presence of SC benzene solvent and the introduction of DTBP greatly accelerate the dealkylation related lumped reactions. Benefiting from improved diffusion environment, the superimposed radicals resulting from DTBP introduction effectively participate in visbreaking and selectively accelerate dealkylation. After only 10–20 min of reaction at 380 °C, visbreaking products with satisfactory viscosity reduction, reduced asphaltene content and improved light component yield can be obtained. • Initiator-introduced heavy oil visbreaking in supercritical benzene was examined. • DTBP introduction results in rapid superposition of alkyl C radicals. • The presence of supercritical benzene accelerates visbreaking. • Superimposed radicals effectively participate in visbreaking in SC benzene. • Best upgrading is obtained in DTBP-introduced visbreaking in SC benzene. [ABSTRACT FROM AUTHOR]