In this thesis an electrical model of On-Board Computer (OBC) using commercial-off-the-shelf (COTS) components for the first Finnish small satellite, Aalto-1, is designed and built for testing the on-board hardware and software. Both the theoretical and practical aspects of designing a reliable OBC are investigated in this work. The Aalto-1 OBC is designed to provide a platform for Command and Data Handling System (CDHS) that interfaces with other subsystems of the satellite and controls their operations. The Aalto-1 OBC is based on ATMEL ARM9 processors and runs a commercial operation system that is able to boot from several devices.When designing equipment to spacecraft, several aspects should be accounted for, and the design must be highly reliable. The methods for increasing the reliability of OBC are introduced, and the applicable methods are implemented in the OBC design. The components are selected and the electrical model of OBC is built. The functionality of the model is tested in thermal cycling chamber to qualify the design and the selected components to be used in the engineering model of OBC.