A numerical study using a computational fluid dynamics approach is conducted on the control effectiveness of a high-speed underwater vehicle with an X-stern configuration. The calculation method is verified by comparing its results with the calculation results and measurements of control derivatives of an underwater vehicle with a cruciform stern configuration. The results of the study show that the control effectiveness of a high-speed underwater vehicle is improved by about 40% in terms of control derivatives, when the cruciform stern configuration is replaced with an X-stern configuration.