Information on the degree of deactivation is very important for predicting catalyst life, from which the catalyst design could be further improved. The CoMo/Al2O3 catalysts, used for the hydrodesulfurization of light oil, are known to have a commercial operation time of around two years. Our goal was to establish the accelerated precoking conditions for obtaining a desired degree of deactivation for CoMo/Al2O3 catalysts. The effect of coking temperature on the catalytic activity for thiophene hydrodesulfurization was investigated for three coke precursors. Precoking with heavier precursors resulted in higher coke content and higher aromaticity as expected, i.e., in order of anthracene>naphthalene>cyclohexene. It is suggested that the established conditions of precoking for the commercial catalysts could provide a quantitive reference for evaluating the lifetime of any commercial catalysts.