Driver's drowsiness is one of the major causes of serious accidents in road traffic. Thus, special effort in searching for better assistant technology has been paid. However, several existing approaches fail to work effectively as the head of a drowsy driver is usually in slanting state. Moreover, the shaking of vehicle or the driver's winking even makes the problem much more complicated. Anyway, head bend posture also signifies a drowsy state. Consequently, this paper proposes a novel approach by considering head nodding behaviour as an input in our detection model. After detecting a human face, some significant facial features are extracted; then, they are used to calculate the predetermined optimal parameters; finally, drowsiness is evaluated based on these thresholds. In our empirical experiments, the proposed algorithm can successfully and accurately detect 96.56% of cases.