The exploitation of thorium fuel in present and future reactors is essential for sustainability of nuclear energy. In this work the performance of thorium fuel in fast reactors as alternative fuel for the standard natural uranium fuel is analyzed. The European Lead cooled Fast Reactor (ELFR) is an advanced system that has been chosen for research and development by generation-IV (GENIV) initiative. A representative fuel assembly of this fast spectrum concept was modeled using MCNPX transport code. The performance of thorium-based fuel is investigated and analyzed in the ELFR concept using the modeled assembly. Also, for the sake of comparison with the studied case, the reference (uranium) case for the modeled ELFR assembly has been also simulated. The analysis and comparison included a number of important neutronic and safety parameters such as the infinite multiplication factor (k-∞), neutron yield, energy of neutrons causing fission, effective delayed neutron fraction (β-eff), and Doppler effect. The performance analysis has showed that using thorium has a positive impact on evolution of the multiplication factor and can help in degrading the plutonium vector.