A method for predicting lane line is provided in the present invention, in which the method is capable of predicting and reconstructing the unidentified lane line according to a predetermined geometry relationship based on the recognized lane line data previously while one side of lane line is unable to be identified. In one embodiment, a lane predicting relationship formula is formed by describing the geometry relationship between the two identified lines defined the lane. When the one side of line of the lane is unable to be identified, a new predicted line can be reconstructed for representing the unidentified line of lane according to the lane predicting relationship curve. In another embodiment, a lane departure warning system utilizing the foregoing method is provided for monitoring vehicle moving status and alerting the driver so as to enhance the vehicle safety.