Abstract One of the most important issues in nuclear reactor operation and its designing is considering the interaction between thermal hydraulics and neutronics physics because there is an important relationship between the states of fluid and neutron spectrum and power distribution. In this research, the MCNP4C and COBRA-EN nuclear codes were coupled with each other to precisely analyze the fuel assembly of the light water reactor core. This coupling was carried out using iterative processes between the linked neutronic and thermal-hydraulic codes applying successive procedures while the desired convergence was made in both. The newly designed code was checked for three test problems, and the obtained results showed the improvement of the computations procedures by the developed code.