Abstract: The effects of three free radical initiators including azodiisobutyronitrile (AIBN), Di-tert-butyl peroxide (DTBP) and iodine (I₂) on the hydrocracking of Kelamayi vacuum gas oil (KVGO) and Gudao vacuum residue (GDVR) were investigated in an autoclave-style reactor. The effects of the amounts of initiator and catalyst added and the reaction temperature on the hydrocracking were examined under an initial H₂ pressure of 7.0MPa (reaction time, 1h). The results indicated that the conversion of heavy oil, the yield of light distillates, and the extent of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) are improved largely by the addition of the initiators for hydrocracking. By adding 5000×10^-6 or 10000×10^-6 initiator in the feed, the yield of light distillates can be increased by 10% or 13% and the extent of HDS and HDN is increased by about 11% or 7%, respectively. DTBP and I₂ are better initiators in the hydrocracking than AIBN. An analysis of the cracking-products reveals that the addition of initiators is favorable for the formation of the primary crackingproducts with VGO or vacuum residues as the hydrocracking feed.