Abstract: The core code of COSINE package uses NEM to solve diffusion equation and NEM is based on the node homogenization theory. In the AP1000 core design process, there are some reasons which cause material heterogeneity in the axial direction, such as the fuel assembly dividing into several areas with different enrichments, the arrangement of spacer grid and the insertion of control rod, and it is needed to be rehomogenization. The re-homogenization can bring error into NEM calculation result because of some hypothesis. The directive phenomenon of the error is control rod cusping effect. Sub-node technology was proposed to solve the material heterogeneity in the axial direction problem in this paper, and the differential worth of control rod was calculated. The results show that the control rod cusping effect greatly reduces because of the sub-node technology.