Electronic circuits are designed as a hierarchical structure of functional blocks. Each functional block is decomposed into sub-functional blocks until the individual parts are reached. In order to formalize knowledge of these circuit structures and functions, we have developed a type of logic grammar called Extended-DCSG. The logic grammar not only defines the syntactic structures of circuits but also defines relationships between structures and their functions. The logic grammar, when converted into a logic program, parses circuits and derives their functions.