This paper presents the application of the object-oriented concept in the development of the software for OSI Application layer protocols. In the development of communication system, it is important to implement those protocols with high maintainability and high extensibility to correspond various requirements from user applications. Therefore, high modularity is required in the software for them. We interpret the modularity as three independencies-independency between protocols, independency between protocol procedure and protocol data, and independency between activated invocations. To this end, we designed a software structure model for OSI Application layer protocols using the object-oriented concept. According to this model, we developed software for several OSI Application layer protocols such as FTAM (File Transfer Access and Management) , TP (Transaction Processing) etc. using our own original object-oriented language. We confirmed that this model engaged high maintainability and high extensibility to accommodate those protocols and improved the modularity of the software.