Abstract: The authors developed a two directional seam tracking system with a laser sensor.It makes use of one of the groove edge as the basic tracking line.An "on-off" control systm is adopted and only one laser beam transmitted by optical fibres is used effectively for both transverse and height automatic tracking.This system is adaptable for butt and lap joints of various kinds of metals in single-pass or multipass welding.The sensor is light in weight,compact and is able to search for the tracking edge within a much wider area of the plate before welding started.