A systematic study has been conducted in a trickle-bed reactor using a commercial NiMo/Al2O3 catalyst to understand the effects of different variables such as H2S concentration in the feed by adding different amount of butanethiol, reaction pressure, temperature, liquid hourly space velocity (LHSV), and H-2/feed ratio on the hydrodenitrogenation (HDN) of typical basic (acridine) and nonbasic (carbazole and 9-ethylcarbazole) nitrogen compounds present in heavy gas oil. The HDN conversion of basic compound was higher than that of nonbasic compounds at all butanethiol concentrations (0-4 wt %) in the feed. The HDN conversion of acridine was 9899 wt % at 355-400 degreesC, whereas, with an increase in temperature from 355 to 400 degreesC, the conversion of carbazole and 9-ethylcarbazole increased somewhat from 92 to 95 wt % and from 94 to 97 wt %, respectively. Pressure (1120-1420 psig) had no effect on the HDN conversion of basic and nonbasic nitrogen compounds. Also, an increase in LHSV did not have a significant effect on the conversion of acridine and 9-ethylcarbazole. However, the conversion of carbazole increased from 92 to 99 wt % with a decrease in LHSV from 2 to 0.5 h(-1). The increase in H-2/feed ratio from 200 to 800 mL/mL caused a significant increase in conversion of carbazole from 90 to 98 wt %. The present studies showed no steric hindrance effect of the alkyl group present in 9-ethylcarbazole.