A messenger program in this paper was designed and implemented using Use Case Diagram, Class Diagram, Activity Diagram, and Sequence Diagram. We selected total 7 major functionalities such as PC server, login, friends management, chatting, file transfer, account management, etc. The followings are various implementation results from this paper. First, developers could easily understand the desired product and programming flow from using UML Diagrams. Second, PDA users could communicate with developers for a long time without time constraints. Third, there was high usability of UML Diagram in the phase of objected-oriented analysis. Forth, it was easy to update and maintain a program.