On-line roughness measurement of a surface with one-dimensional manufacturing marks is difficult to implement. For example, a contact stylus-type inspection method often does not perform very well or fails without any prior knowledge of the mark distribution on the surface. In this paper, we propose an on-line surface roughness measurement method based on laser light scattering, which is very effective for roughness measurement of one-dimensional manufacturing surfaces. The surface roughness is obtained from the spatial distribution of the scattered light intensity. The measurement setup has a very simple configuration, which consists of a CCD sensor, a collimated diode laser and an expander. The orientation of the spatial distribution of the scattered light intensity from the surface, which depends on the surface orientation, is detected by the CCD sensor, and then the mark direction can be readily determined from image processing. After that the root-mean-square (RMS) height of the surface roughness is extracted by means of image processing of the scattered light distribution in the direction parallel to the manufacturing mark, rather than in the direction perpendicular to the mark which is often followed by other measurement probes. The experimental tests show that the non-contact method has great potential for on-line surface roughness measurement.