Abstract: In order to calculate and study thermal-hydraulic characteristics of natural convection assembly (NCA) of sodium cooled fast reactor (SFR), a sub-channel code was developed in the paper. Based on a single assembly model with 61 rods, the applicability of the developed sub-channel code to NCA was validated by comparison with COBRA code. Based on a multi-assembly model, the automatic flow distribution model and the inter-assembly heat transfer model of the code were preliminarily validated. This code will provide an effective design and analysis tool for NCA of SFR.