Thermal control is one of the key subsystems of CubeSats, playing a critical role in ensuring the reliable operation of onboard electronic components and enhancing mission reliability. This study evaluates the performance of a passive thermal control system designed to reduce power consumption and eliminate the need for a battery heater in a CubeSat. First, a thermal model of the satellite was developed in a specialized thermal analysis environment and validated against a benchmark analytical problem, yielding a discrepancy of approximately 1.16% from the analytical solution. A grid independence study further confirmed the accuracy of the numerical model. Thermal analysis under the worst-case hot scenario revealed that the temperatures of several components exceeded their allowable operating limits. To address this issue, aluminum heat sinks, heat spreaders, thermally conductive interfaces, and conductive paths to radiator panels were incorporated into the design. The proposed passive thermal control system reduced the temperatures of the communication board and battery cells by approximately 60% and 56%, respectively, while maintaining all components within their permissible operating temperature ranges. Furthermore, analysis under the worst-case cold scenario demonstrated that the battery temperature remained within the acceptable range without requiring a battery heater.