Abstract: A method for analysis of boiling range distribution of hydrocarbon molecular composition in diesel by gas chromatography-time of flight mass spectrometry (GC-TOFMS) was established. Carbon number distributions, average molecular weights and structures of hydrocarbon in different boiling ranges were studied. It is found that the average molecular weight of distillation range was influenced by aromatic content and degree of isomerization. The lower the aromatic content and the higher the isomerization degree is, the higher average molecular weight of the boiling range is. The boiling range distributions of hydrocarbon molecular compositions in diesel before and after hydrotreating were investigated. For hydrotreated diesel, boiling range distribution of saturated hydrocarbon is basically unchanged, but its concentration increases. Boiling range distribution of aromatic remains the same in low distillate fraction, whereas there is a concentration decrease in high distillate, which results in a higher average molecular weight of product than that of raw material diesel in high distillate fraction..