Vehicle-to-vehicle　distance　calculation　has　a　great　significance　to　driving　assistance　and　estimation　of　traffic　condition.　In　this　paper,　we　present　an　on-board　video-based　method　about　calculating　distance　gap.　The　method　is　mainly　divided　into　three　major　stages.　At　first　stage,　an　Adaboost　cascade　classifier　using　Haar-like　features　of　sample　pictures　is　used　to　detect　preceding　vehicles.　At　second　stage,　a　fusion　algorithm　combining　Maximally　Stable　Extremal　Regions　(MSER)　for　far　vehicles　with　vertical　texture　method　for　close　vehicles　is　applied　to　locate　license　plate.　At　the　third　stage,　distance　gap　is　calculated　according　to　the　pixel　height　of　plate　and　the　proportion　of　plate　pixel　height.　Experimental　results　in　this　paper　showed　excellent　performance　of　the　method　in　calculating　distance　gap.