The objective of this research is to find an optimal ratio of rare earth elements (RE), lanthanum (La) and cerium (Ce), as promoters of Co-based Fischer-Tropsch synthesis (FTS) catalysts to achieve maximal diesel yield. These synthesized rare-earth promoted catalysts were characterized with Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD) and hydrogen temperature programmed reduction (H-2-TPR).The effects of La/Ce molar ratio on the FTS performance of silica gel supported cobalt based catalysts were investigated in a fixed bed reactor at temperatures of 220 and 240 degrees C. The evaluation tests show that the catalysts promoted by composited rare-earth (CRE) with appropriate molar ratio La to Co have better FTS performance than unpromoted and individual rare-earth (IRE) promoted catalysts. The results show that the CRE promoted FTS catalysts are promising. The CO conversion is increased significantly and the selectivity for methane and C-2-C-4 is decreased while the selectivity for long chain hydrocarbons is greatly increased (C-5(+) increased from 70.36 to 80.25% at 220 degrees C and from 69.94 to 78.02% at 240 degrees C, respectively). Among all CRE modified catalysts, based on the result of distribution of C-12-C-18 (weight% of hydrocarbons), Co-lLa2Ce/S.G (n RE: n Co = 0.1 and n La: n Ce = 1:2) is proven as a highly efficient catalyst for production of diesel rich synthetic oil from coal-derived syngas. (C) 2015 Elsevier B.V. All rights reserved.