A two-stage concurrently and countercurrently operated fixed-bed reactors were investigated to remove sulpfur and aromatics in producing ultra-clean diesel fuel. Under similar operating conditions, the sulfur concentration in the produced oil through countercurrent operation is much less than under concurrent flow. In a typical run with countercurrent flow at 6.0 MPa and 360 degreesC, the sulpfur content could be removed from 13,841 to 5.7 mug/g with nitrogen from 457 to 1.8 mug/g and the total aromatics from 41.4% to 26%. One-dimensional heterogeneous model was established to simulate the distribution of all the components involved along the reactor length in the gas and liquid phases, which revealed that the low H2S concentration at the reactor exit region under countercurrent flow is critical to the ultra low sulphur extent in the produced oil. (C) 2004 Elsevier Ltd. All rights reserved.