To evaluate the surface quality of ring core spun yarns, we set up a quality measurement system that operated on the basis of a CCD camera along with specific software programs that recognised the surfaced core filaments. The acquired images were transformed to binary states and filtered through a moving window. By defining a quantitative index by means of the number of defects per length of the specimen, the effects of the processing variables such as production speed and traveller mass on the surface quality of core spun yarns were investigated. Results demonstrated that the new measuring system could be applied effectively to assess the core coverage of yarns. There existed some optimal levels of yarn production speed and traveller mass that led to a minimum occurrence of the surface defects of core spun yarns.