Today, complex, high-quality computer based software are required to be built in very periods. So, it has motivated utilizing off-the-shelf software components for rapid development in the field of software development. Computer based software Engineering is a process that emphasizes the design and construction of computer based systems using software components. The goal of component based engineering is to increase the productivity, quality and time to market in software development. Component based software applications are expected to have high reliability as a result of deploying trusted components. In this paper, an approach is presented for system reliability assessment if the component reliability is known. This includes probability of failure of the component and its usage ratio to find the system reliability. The reliability of a software component is a probability prediction for failure free execution of the component based on its usage requirements. The component severity analysis is also done to support a software development organization to obtain the best reliability improvements.