The objective of the present study is to develop abetter hydrodesulfurization (HDS) kinetic model on the basis of a fundamental and comprehensive understanding of the different HDS reactivities of various sulfur,compounds in middle distillates and the inhibition effects of the coexisting nitrogen and aromatic compounds. Five middle distillates blended from straight run gas oil, heavy gas oil, and coker gas oil were hydrodesulfurized over a commercial catalyst at 300 degrees C, 4.0 MPa H-2 pressure, a H-2/oil volume ratio of 200, and different liquid hourly space;velocities. Identification and quantification,of Sulfur compounds in the feedstocks and their hydrodesulfurized products were conducted by using GC-PFPD to classify the sulfur compounds into four groups according to their molecular structures and reactivities. It was found that the HDS of each sulfur group follows pseudo-first-order kinetics. Correlation of the obtained first-order rate constants with both the nitrogen concentration and the feedstock density indicates that the effects of both on the rate constants can be expressed, by a linear function of them. A four-lumping first-order-kinetics model including effects of the nitrogen concentration and density was developed, which can predict the total sulfur concentration in the HDS product from different middle distillates and is useful to understand how the composition and properties of the middle distillates influence their HDS reactivity and kinetics.