This paper presents critical steps in building client/server application with UML and Visual Basic, which are derived from the implementation case of a typical order processing system. To begin with, we briefly review the software architecture, the diagrams and the object-oriened building process in the UML. In the inception phase, it is critical to define the project charter, to draw use case diagrams, and to construct a preliminary architecture of the application. In the elaboration phase, it is critical to identify classes to be displayed in the class diagram, to develop user interface prototypes for each use case, to construct sequence diagram for each use case, and finally to design an implementation architecture. Steps to construct implementation architecture are given. In the construction phase, it is critical to design both the database and components. Steps to design these components are described in detail. Additionally the way to create the Internet interface is suggested.