A High performance gel permeation chromatography(GPC),evaporative light-scattering detector(ELSD) method were applied to determine the relative molecular weight and its distribution of vacuum gas oil(VGO),adopting linear calibration with kown relative molecular weight calibration standard and complicated mathematics operation. Also the precision of the technique was evaluated and the relative molecular weight between GPC and VPO was compared. The results show that the relative error is less than 1%, comparison with traditional method of VPO the analysical rate is promoted five times, the sample oils less to 1/10. The technique is accurate, fast and high level of automatic operation. 