Over the past decade, numerous software managers and engineers have been concerned with measuring the size and complexity of software systems. Function point analysis technique is one of the most popular software sizing techniques. A reasonable software development plan through cost and time estimation should be a prerequisite for the successful project at the beginning stage of the project. It is generally known that software size measurement is useful for this kind of estimation and the function point analysis technique would be more effective than the others. However, it is difficult to apply the technique to object-oriented methodology widely used in the software industry. Thus, the purpose of this study is to present a case study on how to apply function point analysis technique to sizing of the software systems based on UML. The results of this study can be useful to managers and engineers.