Core spun yarn is understood as a cross-sectionally structured composite yarn in which the core strand is wrapped by staples. The quality of the core spun yarn is thus determined by the structural characteristics, if the core filaments are firmly located in the center of the yarn and well covered by the sheath-staples. A variety of core spun yarns can be attained by various combinations of sheath staples and core filaments, which leads to a better mechanical, thermal, or physiological performance of the yarn. Applying the ring spinning principle, however, to manufacture the core spun yarn often causes defects in the yarn; for example, the aesthetic defects due to the appearance of the core filaments on the yarn surface. In this study we conceived and constructed a measuring system that operates on the machine vision along with programs for recognizing the surface defects and monitoring the measuring process. We checked the feasibility of the new measurement system. Experimental trials demonstrated that the wrapping defect measurement system gave reproducible and trustable results. The twist effect on the surface defects of the core spun yarn, which is closely related with the probable occurrence of the core filaments on the yarn surface, can be described by a wrapping defect index and it revealed that there is a twist level that provides the yarn with the minimum number of defects on the surface.