In this paper,a method was established to determine the boiling point distribution of hydro-carbon types in diesel.The diesel sample was separated into the saturate and aromatic fractions by meansof solid-phase extraction (SPE),and each fraction was analyzed by GC/FID-EIMS.According to therelationship between boiling point and retention time of n-paraffins in the chromatogram,the percent-ages of saturates and aromatics at each temperature interval were calculated.According to the averagemass spectra of the saturate and aromatic fractions at each temperature interval,the hydrocarbon typesof the sample were identified through summation of characteristic mass fragments.Using this method,the changes in composition of diesel during hydrotreating process were studied.The results showed thathydrogenation of aromatics is the main reaction during the hydrotreating process.The more rings thearomatics have,the easier the hydrogenation reactions would take place.The aromatics were convenedinto cycloparaffins via the hydrogenation and saturation process,leading to an increase in low boilingpoint fractions in the hydrotreated oil. In this paper, a method was established to determine the boiling point distribution of hydro-carbon types in diesel. Diesel sample was separated into saturate and aromatic fractions by means of solid-phase extraction (SPE), and each fraction was analyzed by GC /FID-EIMS.According to therelationshipbetween boiling point and retention time of n-paraffins in the chromatogram, the percent-ages of saturates and aromatics at each temperature interval were calculated. According to the average mass spectra of the saturate and aromatic fractions at each temperature interval, the hydrocarbon types of the sample were identified through summation of characteristic mass fragments. Using this method, the changes in composition of diesel during hydrotreating process were studied. The results showed that hydrogenation of aromatics is the main reaction during the hydrotreating process. more the more the hydrogenation reactions would take place. aromatics were convenedinto cyc loparaffins via the hydrogenation and saturation process, leading to an increase in low boiling point fractions in the hydrotreated oil.