Abstract: In this paper, URANS (Unsteady Reynolds Averaged Navier Stokes) methodology was applied to the prediction of turbulent flow inside tight lattice rod bundles with different P/D ratios, i.e. from 1.001 to 1.2. The turbulent flow with Reynolds number ranging from 5 000 to 215 000 was investigated by analysing the bulk velocity, wall shear stress and turbulent intensity distributions in the subchannel. The results show significant effect of P/D on stream wise velocity and wall shear stress in very tight lattice (P/D<1.1). The results also indicate that the mixing factor (Y) has an inverse proportion relation to δ/D, even though δ/D is smaller than 0.011. When Reynolds number reaches certain value in tight lattice, the bulk velocity and wall shear stress are less sensitive to Reynolds number.