The present invention provides a lane departure warning system including an image sensing device, an identification module and a determination module. The image sensing device generates a video. The identification module identifies a plurality of lane lines of at least one lane on which a vehicle is currently driven from the video, to generate an identification result, wherein the identification result is generated based on specific borderlines of the lane lines. The determination module determines a determination strategy according to vehicle speed information, and generates a determination result according to the determination strategy and the identification result. The lane departure warning system determines whether to issue a warning according to the determination result.