Software is increasingly being used to handle various functions in safety-critical systems. A large number of hazards are known to be caused by the software that controls it. Therefore investigating safety factors in software to identify and assess potential hazards, is essential in improving system safety and reliability. We propose a safety analysis technique of FBDs, which is a programming language commonly used in safety-critical embedded software development. It is currently being used in the development of the Reactor Protector System(RPS) of the nuclear power plant system which is being developed at KNICS in Korea. Our approach is a combined template-based fault tree analysis technique. We combine two fault tree views in our templates so that our analysis technique is easily applicable and more complete. The templates contain information on the possible faults that can occur in FBDs and the propagation paths, and are defined for each function block in the FBDs. Our work provides templates and application procedures to the fault tree analysis of FBDs. We introduce our approach and show it applied to the RPS, which demonstrates our systematic technique is useful and applicable to real-world systems.