Abstract: Coupled neutronics and thermal-hydraulics calculation can enhance the computational accuracy in reactor simulation. With the development of computational capability, the coupled Monte-Carlo neutron transportation and thermal-hydraulics calculation plays an important role in reactor core design and reactor safety. This paper gives a short introduction of the Monte Carlo code JMCT, the sub-channel code COBRA-EN and their coupling scheme. The coupled codes are applied on the 33 fuel bundles model and the UO2 assembly simulation. The calculated results are discussed, especially, the convergence criteria and quantification of correlation between statistical uncertainty and coupled error.