That vehicles travel on a curve with excessive speed tends to skid or roll over. This study presents research in video recognition technology of lane and its application in traffic early safety alert system, which improves traffic safety. In our study, we use a monocular camera to acquire the image of the road ahead. Techniques such as gray-scale transformation, filtering, edge detection and curve fitting are used for the recognition of the lane line and curve fitting. Then radius of curvature of the road ahead will be extracted and then the maximum safe speed, which is to be compared with the speed of the vehicle in safety alert system. Experiments show that this method can accurately calculate the radius of the curve ahead and timely give alert message when the vehicle’s speed exceeds safe speed.