The Fischer-Tropsch synthesis of unpromoted and nano-sized iron oxide supported on delta-Al2O3 was investigated using a fixed-bed reactor. The catalysts prepared from pre-synthesized iron oxide with varying particle size (2-12 nm) showed much higher catalytic activities than the one prepared by using conventional impregnation method. The best results for CO conversion were obtained when the catalyst had Fe particle size of 6.1 nm. With an increase in particle size, the reduction degree and C-5+ selectivity was increased, whereas CH4 selectivity and the uptake of adsorbed CO were decreased. Turnover frequency (TOF) at 300 degrees C was increased from 0.02 to 0.16 s(-1) when d(Fe-0) was increased from 2.4 to 6.2 nm, and then it remains almost constant up to a particle size of 11.5 nm. Particle sizes of prepared iron oxide were analyzed by XRD and TEM, and the reduction behaviors of Fe/Al2O3 catalysts were studied by H-2-TPR. The effective iron size, metal dispersion and reduction degree of Fe/Al2O3 catalysts were measured by CO chemisorption and O-2 titration. (C) 2010 Elsevier B.V. All rights reserved.