Upgrading of Estonian shale oil heavy mature fraction boiling over 320 degrees C and characterized by high content of high-polar and heteroatomic compounds (46.6 %) and by low content of aliphatic hydrocarbons (6.6 %) was carried out in an autoclave at 37 degrees C using Co-Mo and Ni catalysts. Both catalysts gave high recovery (over 90 %), but when Co-Mo catalyst was used more volatiles with higher conversion level and less coke were obtained. The content of high-polar and heteroatomic compounds in hydrogenisate was decreased twice and that of aliphatic hydrocarbons was increased just as much, and as a result also other properties of heavy mazute were significantly altered.