The security of smart phones is increasingly important due to their rapid popularity. Mobile computing on smart phones introduces many new characteristics such as personalization, mobility, pay-for-service and limited resources. These features require additional privacy protection and resource usage constraints in addition to the security and privacy concerns on traditional computers. As one of the leading open source mobile platform, Android is also facing security challenges from the mobile environment. Although many security measures have been applied in Android, the existing security mechanism is coarse-grained and does not take into account the context information, which is of particular interest because of the mobility and personality of a smart phone device. To address these challenges, we propose a context-aware usage control model ConUCON, which leverages the context information to enhance data protection and resource usage control on a mobile platform. We also extend the existing security mechanism to implement a policy enforcement framework on the Android platform based on ConUCON. With ConUCON, users are able to employ fine-grained and flexible security mechanism to enhance privacy protection and resource usage control. The extended security framework on Android enables mobile applications to run with better user experiences. The implementation of ConUCON and its evaluation study demonstrate that it can be practically adapted for other types of mobile platform.