Abstract: Deep-cutting operation of vacuum distillation is limited by its investment and energy consumption and its coking tendency in vacuum furnace. In this paper, the coking tendencies of residue fractions（>565℃）were connected with their amount of toluene insoluble (TI), the precursor of coke, formed during heating. The effect of temperature and residence time in the furnace on the amounts of TI of heavy fractions was tested to get the critical curve of anti-coking for the fractions with different properties. It is discovered that high temperature and longer staying time in the furnace will aggravate the formation of TI and that the fractions from different base crudes have their own special critical curve of anti-coking. The test results indicate that the amount of TI can be used for judgment of coking tendency of the heavy fractions.