Abstract: By the degree of synchronous codes, some combinatorial characteristic of synchronous codes and prefix synchronous codes were studied. The completion of these codes was given. For synchronous codes, there is a word with degree 1. All words beginning and ending by this word are a free submonoid. Using the base of this free submonoid, the completion of synchronous codes was constructed. For prefix synchronous codes, an unborderd word with some special characteristic was found. The completion of a prefix synchronous code was made from the code itself. The prefix root of the words ends by this word and doesn't begin by the code words.