Abstract: Based on the SH/T 0659 method for analyzing saturate distillate in gas oil and GC-MS technique, a new method was established to measure the total content of normal paraffin, iso-paraffin, and the carbon number distribution of normal paraffin in hydrocracking tail oil. The GC-field ionization time-of-flight high-resolution mass spectrometry (GC-FI TOFMS) was used to calculate the carbon number distribution of the sample and then the carbon number distribution of iso-paraffin. Three hydrocracking tail oil samples at different conversion levels were studied by the new method to investigate the impact of conversion depth on the paraffin structure of tail oil. The results show that n- and iso-paraffin and total paraffin content are increased with the degree of conversion, while the ratio of normal one in total paraffin decreases. The carbon numbers in n-and iso-paraffin as well as in total paraffin are normally distributed. At high conversion level, more low carbon n- and iso-paraffin is in the tail oil, while high carbon paraffin becomes more at lower level.