It is dangerous for drivers to make a call while driving, as it could easily divert the drivers' attention. In this paper, we present a novel method to detect the driver use of mobile phone based on an in-car camera. The in-car camera is mounted on the front windshield to capture the video of the driver during driving, and an activity parsing algorithm is employed to identify whether the driver is using a mobile phone. We decompose the phoning activity into three actions and use the And-Or Graph (AoG) to represent the hierarchically compositions of the phoning activity and the temporal relationship between the actions. An online parsing algorithm for AoG based on Early's parser is implemented to parse the video and detect the driver use of mobile phone. Experiment results on collected video data set shows that the proposed method can detect the driver use of mobile phone precisely. This method could be used as a supplement of the safety device inside the vehicle.