An effect of organic nitrogen compounds on deep hydrodesulfurization catalyst (CoMo/Al2O3) has been carried out using micro-flow reactor at 5 MPa hydrogen pressure, 1.5h(-1) liquid hours space velocity, and 330 to 380 degrees C temperature. The experiments were conducted in two steps; at first, prepare nitrogen free Kuwaiti straight run gas oil (SRGO) while in the second step is to test hydrotreating (HDT) catalytic activities using synthetic diesel feed to achieve ultra-low sulfur diesel (ULSD). Hence the role of nitrogen compounds and their effect on HDS activity was studied with nitrogen-free SRGO and spiked (added indole and quinoline), which indicated an apparent tendency of strong inhibition effect by basic nitrogen on deep HDS process. Indeed, inhibition and deep HDS conversion are also dependent on the H-2 partial pressure and its dissociative species (i.e., H delta+ and H delta-). It is expected that basic nitrogen compounds can easily consume H delta+ ions during hydrotreating which is assumed to be the rationale behind the stronger inhibition of the hydrogenation function. It was found that quinoline has a more noticeable adverse effect than indole, particularly at a lower temperature. HDS activity results indicated that the adsorbed nitrogen species showed more acceptable behavior at a lower temperature while at high-temperature inhibition effect is not very significant, which could be because of the adsorption nature of nitrogen species.