Abstract: The effects of four free radical initiators including azodiisobutyronitrile (AIBN), di-tert-butyl peroxide (DTBP), sulfur(S) and iodine(I₂) on the autoclave-style visbreaking of three vacuum residues were investigated under the reaction conditions of 390℃～410℃ and the initiator dosage of 0~3000×10^-6 according to free radical chain reaction mechanism of thermal-cracking of vacuum residues. The improvement mechanisms of the free radical initiators were studied by analysis of the viscosity and chemical composition of the processed oil samples. The results show that the free radical initiators improve the visbreaking of different-base vacuum residues， in which the order of improvement is LHVR≥GDVR>DQVR. The improving effect, which is dependent on the chemical-physical character of the processed residues, could be outstanding at lower reaction temperatures. The effect of I₂ or S under the same reaction condition is twice higher than that of AIBN or DTBP, which could be attributed to the circulated initiation of ‘active atom’ originated from the inorganic HI or H₂S. Compared with S, the improvement of iodine is better.