Abstract: Investigations on the thermal-hydraulic behavior in the SCWR fuel assembly have obtained a significant attention in the international SCWR community.However,there is still a big deficiency in knowl-edge and prediction of the heat transfer in supercritical fluids.In this paper,CFD analysis is carried out to study the thermal-hydraulic of supercritical water flows in sub-channels of a typical SCWR fuel assembly using commercial CFD code CFX 5.6.Effects of various parameters,such as flow parameters,boundary con-ditions and pitch-to-diameter ratios on the mixing phenomenon between sub-channels and heat transfer,are investigated.The mixing phenomenon due to cross flow between wall sub-channels is much stronger than that between regular sub-channels.The turbulent mixing in tight lattice（P/d=1.1）is lower than that in wide lattice（P/d＞1.1）,whereas,the effect of pitch-to-diameter ratio on the turbulent mixing is slight at P/d＞1.2.It also shows unusual behavior in the vicinity of the pseudo-critical point and further investigation is needed.