Driver fatigue is the major cause of traffic crashes and financial losses. This paper presents an advanced computer vision and mobile technology using smartphones to monitor visual indicators of driver fatigue, allowing the possibility of making fatigue detection systems more affordable and portable. This technology uses the front camera of a smartphone to capture images of drivers, and then uses advanced computer vision algorithms to detect and track the face and eye of the drivers. Head nod, head rotation and eye blinks are then detected as indicators of driver fatigue. A simulated driving study showed that drowsy drivers differed significantly in the frequency of head nod, head rotation and eye blinks, compared to when they were attentive. The smartphone-based fatigue detection technology may have important applications in reducing drowsiness-related traffic accidents and improving driving safety.