To accommodate the frequent changes of user requirements and operating environments, software systems have to be much more flexible. Fortunately, modeling variability is just a realistic and efficient approach to controlling variability and implementing software reuse, which does well not only in identifying and expressing variability, but also in assisting the management of variability evolution. This paper proposes an approach to modeling variability in product family. In this approach, variabilities of system behaviors are modeled through extended use case models, while variabilities in functionality and quality are captured by feature models, and above all, both the models adopt consistent mechanisms to model variability. Summarily, the whole modeling process of the approach is discussed systematically through a real software family on mobile phone.