The IEC 61499 standard has become one of the key approaches to building distributed component-based control systems in industrial automation. The problem of adoption of this standard in industrial practice is often associated with incompletely defined semantics of functional blocks (FB), which are main design artifacts of the standard. In this paper we propose formal (operational) semantics of IEC 61499 FB using transition systems (by example of FB systems operating in accordance with the cyclic execution model). The proposed FB operational semantics is most convenient for the formal verification of FB systems on the basis of model checking as it describes the direct transitions between states. A limitation of this approach is the need to map hierarchical FB systems to flat models. The technique of code generation used for transforming FB transition systems to SMV models is briefly discussed followed by a simple case study.