Formal Methods have changed software engineering development process. They have been used in implementation of software for safety-critical environments. Formal specification languages have good properties for describing a system. ACT-ONE and Communicating Sequential Process (CSP) are two specification languages we have discussed on them in this paper. This paper compares them from different aspects such as their styles, special symbols, databases, and combination with other languages. As future work, we will present a new formal model for a distributed system in our next research.