Abstract: A deasphalted oil (DAO) was hydroisomerized in a flow type fixedbed reactor over a commercial catalyst at 9.0MPa, 380℃ and LHSV 0.8h-1. The products were distilled and separated by true boiling point distillation method. Carbon number distribution and hydrocarbon type of various cuts were determined by normal or high temperature GC-MS. It was found that the viscosity and the yield increased with the increase of boiling point for the light cuts, while the contents of monocycloalkanes decreased sharply. As for the heavy cuts, their carbon number distributions became wide with the increase of boiling points. Variation of monocycloalkanes in heavy cuts had significant impact on the viscosity index (VI) and pour point of the base oil. VIs and low temperature properties of +380℃ heavy cuts could meet the specifications of API II higher grade lube base oil.