Downloading or buying mobile applications are becoming increasingly popular activities, which would seem likely to affect the daily life of human beings. However, little empirical evidence about the factors, which influence downloading of mobile application is available. This study examines the concepts of attitude, subjective norm and perceived behavior control with reference to the extended theory of planned behavior model to explain why people download mobile applications. We collected 320 valid questionnaires, and used a structural equation model to conduct the empirical study. The results indicated that attitude, subjective norm, and perceived behavior control have a significant effect on the downloading intention of mobile applications. However, the results show that compatibility and self-efficacy have no significant effect on the downloading behavior of mobile applications. Furthermore, this study also finds that network externalities affect user intention and behavior toward downloading mobile applications in an indirect way.