Abstract: The turbulent flow and heat transfer in tight lattice were investigated in the paper. The calculation results were firstly validated with experimental data, then the effects of Re and P/D on the frictional resistance and heat transfer coefficients in tight lattice were analyzed. Both the effects of Re and P/D on the flow and heat transfer characteristics in tight lattice are significant. However, the traditional theoretical models can not predict the effects of Re and P/D on the frictional ictional resistance and heat transfer coefficients. The P/D=1.03 is a critical point. In this lattice, the flow and heat transfer are the safest and most efficient. Both the nuclear power and heat transfer capacity of the system can reach their best.