Natural convection heat transfer from vertical 5 × 5 rod bundles in liquid sodium was numerically analyzed for two types of the bundle geometry (equilateral square array (ESA) and equilateral triangle array (ETA)). The unsteady laminar three-dimensional basic equations for natural convection heat transfer caused by a step heat flux were numerically solved until the solution reaches a steady-state. The phoenics code was used for the calculation considering the temperature dependence of thermophysical properties concerned. The 5 × 5 test rods for diameter (D = 7.6 mm), heated length (L = 200 mm), and L/d (=26.32) were used in this work. The surface heat fluxes for each cylinder were equally given for a modified Rayleigh number, (R_{f}_{,}_{L})_{ij} and (R_{f}_{,}_{L})_{5}_{×}_{5,}_{S}_{/}_{D}, ranging from 3.08 × 10^{4} to 4.19 × 10^{7} (q = 1 × 10^{4}–7 × 10^{6 }W/m^{2}) in liquid temperature (T_{L} = 673.15 K). The values of S/D, which are ratios of the diameter of flow channel for bundle geometry to the rod diameter, for vertical 5 × 5 rod bundles were ranged from 1.8 to 6 on each bundle geometry. The spatial distribution of local and average Nusselt numbers, (Nu_{av})_{ij} and (Nu_{av,}_{B})_{5}_{×}_{5,}_{S}_{/}_{D}, on vertical rods of a bundle was clarified. The average value of Nusselt numbers, (Nu_{av})_{ij} and (Nu_{av,}_{B})_{5}_{×}_{5,}_{S}_{/}_{D}, for the two types of the bundle geometry with various values of S/D were calculated to examine the effect of the bundle geometry, S/D, (R_{f}_{,}_{L})_{ij}, and (R_{f}_{,}_{L})_{5}_{×}_{5,}_{S}_{/}_{D} on heat transfer. The bundle geometry for the higher (Nu_{av,}_{B})_{5}_{×}_{5,}_{S}_{/}_{D} value under the condition of S/D = constant was examined. The correlations for (Nu_{av,}_{B})_{5}_{×}_{5,}_{S}_{/}_{D} for two types of bundle geometry above mentioned including the effects of (R_{f}_{,}_{L})_{5}_{×}_{5,}_{S}_{/}_{D} and S/D were developed. The correlations can describe the theoretical values of (Nu_{av,}_{B})_{5}_{×}_{5,}_{S}_{/}_{D} for the two types of the bundle geometry at S/D ranging from 1.8 to 6 within −12.64% to 7.73% difference.