In this study the effect of catalyst nanoparticle size on the Fischer-Tropsch synthesis (FTS) activity is investigated on iron catalyst. A series of iron oxide particles doped by adding cupper and lanthanum were prepared as a catalyst via precipitation by microemulsion method. Because of the role of iron carbide in FTS reaction by iron catalysts, the iron carbide particle sizes was evaluated after activation and considered as active phase in this paper. The catalytic results indicated that FTS activity increased and passed through a maximum when the iron particle size increased. A size-dependent kinetic model for FTS rates was developed using a thermodynamic analysis method. The model parameters measured by experimental results and the effect of various active sizes on these parameters was evaluated. Calculated size dependent model parameters were employed for evaluation of surface tension energy (sigma) for iron catalyst and size-independent activation energy for FTS reaction. (C) 2013 Elsevier Ltd. All rights reserved.