With the rapid evolution of mobile devices, the usage patterns of applications on these devices also change over time. The topic of user behaviors on mobile devices such as smart phones has attracted much attention in recent years. In this work, we have developed an experimental platform that can log and analyze all user operations on a smart phone that is used freely over a long period of time. The collected data are transmitted to a server periodically when the network is available. Analytic tools such as charting, log query, and real-time log pattern analysis have also been developed to facilitate the study of user behaviors. In this paper, we will illustrate the functionalities of the proposed platform through several examples and a sample case study.