The Fischer-Tropsch (F-T) reaction has been studied over a series of unsupported Fe-Co -Mn nanocatalysts synthesized by the hydrothermal procedure to investigate the effects of Mn addition and Fe/Co mole ratios on the catalytic activity and selectivity in a fixed-bed reactor. The main intention is to maximize the light olefins (C-2(=) - C-4(=)) selectivity by controlling the nanocatalyst composition and process conditions. It was found that the 68% Fe -16% CO -16% Mn nanocatalyst was an optimum nanocatalyst for C-2(=) - C-4(=) olefins production from synthesis gas. The effects of a range of operational conditions on the performance of optimum nanocatalyst were investigated. The results are shown that the maximum olefin/(methane + paraffin) ratio has been obtained over 68% Fe -16% Co -16% Mn sample at an operational temperature of 290 degrees C, the H-2/CO molar ratio of 1/1, the space velocity of 3000 11-1. and 2 bar pressure. Characterization of nanocatalysts was carried out using XRD, N-2 physisorption, TEM, TGA, DSC, TPR, FE-SEM and EDS. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.