A two-dimensional pseudo homogeneous model has been developed to investigate the influence of tube size on the thermal behavior and performance of packed fixed bed reactor for the low temperature Fischer-Tropsch (FT) synthesis over alumina supported cobalt. Velocity, temperature and composition fields are determined by solving the fundamental transport equations in porous media. Special attention was paid to the variation of transport properties with temperature and composition of the gas mixture. High dependency of the thermal behavior On the thermal conductivity of the gas mixture is highlighted, whereas viscosity and heat capacity of gas mixture have very little influence. Moreover for the considered catalyst, simulation results have displayed high heat removal for the millimetric scale with a tube inner diameter below 2.75 min for an extended range of weight hourly space velocity (20-600 g(syngas) min(-1) kg(cat)(-1) T=493 K and P=20 bar). With a millimetric reactor, high CO conversion (X-CO > 90%) is obtained for values of space velocity between 30 and 120 g(syngas) min(-1) kg(cat)(-1). For higher diameter tube than 3.11 mm, thermal runaway occurs and even worse, no convergence achieved due to the very low heat transfer global coefficient and the weak surface/volume ratio, leading to a significant decrease of liquid fuels selectivity and an increase of light hydrocarbon (C-1 to C-4) selectivity up to 14%. To conclude, results from scale-up study with the millimetric scale are outstanding, more than 2900 kg h(-1) m(cat)(-3) of C5+ could be produce after numbering-up 3033 tubes of 10 centimeters in length whereas conventional units (multitubular fixed bed reactors or slurry phase reactors) do not exceed 400 kg h(-1) m(cat)(-3). (C) 2015 Elsevier Ltd. All rights reserved.