Abstract: The effects of ionic liquids on viscosity, hydrocarbon composition and average molecular weight of heavy oils are investigated. The results indicate that ionic liquids have good viscosity reduction property for heavy oils. After reaction with ionic liquids, the amount of saturates, aromatics and resins increase, while asphaltene decreased. This leads to average molecular weight and viscosity of heavy oil decreased. The results also indicate that metal ion modified ionic liquids have catalytic effect on the upgrading reaction of heavy oils.