In the textile industry, composite yarns with multifilament cores are used to impart strength. There are various spinning systems to produce composite core-spun yarns. In this study, to determine the effects of filament fineness on yarn characteristics of composite yarns, polyester filaments with medium, fine and micro fiber linear densities were used as the core portion and cotton fiber was used as the sheath material. Yarn samples were manufactured using a modified ring spinning system with four different yarn counts and constant twist factor (alpha(e)). The effect of filament linear density on yarn tensile properties, unevenness and imperfections was determined. Yarn evenness and tensile properties were compared with 100% cotton ring spun yarn and to each other. When relative amount of core increases, it was observed that composite yarns had improved tenacity and elongation compared to 100% cotton ring spun yarn. Although filament fineness was found to have a significant effect on the CVm % properties, there was no statistical effect on imperfections other than yarn count parameter.