Abstract: The reaction behavior of both catalytic pyrolysis and thermal pyrolysis of HGO (heavy gas oil) and LGO (light gas oil) derived from Canadian synthetic crude oil was investigated in a confined fluidized bed reactor. For the catalytic pyrolysis of HGO and LGO, the yields of total light olefins (ethylene, propylene and butylene) reach a maximum at about 660℃ (33.8% and 35.6%, respectively). The thermal pyrolysis of HGO and LGO is in high extent; the conversions of HGO and LGO at 700℃ reach 66.7% and 76.3%, respectively. For the thermal pyrolysis of HGO, the yield of total light olefins shows a maximum of 27.9% at 680℃. Through comparing the gas product yield of catalytic pyrolysis with that of thermal pyrolysis, it was found that the addition of the catalyst accelerates the formation of ethylene and liquefied petroleum gas (LPG), and simultaneously restrains the formation of methane and ethane. Current results suggested that methane and ethane are mainly produced by the free radical reaction, while ethylene and LPG are produced by both the free radical reaction and the carbonium ion reaction.