Reliability of a system is the measurement of its ability to perform certain functionality over specified period of time. In practice, a system is frequently represented as a structural network such that components are connected together either in series or parallel or combination of these. Many techniques have been developed for the reliability evaluation of independent components such as Reliability Block Diagram (RBD) and Fault Tree Analysis. Accurate reliability analysis is indispensable for safety and mission critical applications. A slight error in the measurement may cause great loss in terms of human casualties and financial cost as well. Higher-order-logic (HOL) theorem proving can be used to conduct the formal analysis of RBDs and FTA.