In this experimental work, the effects of the core-sheath ratio, the filament count of multi-filament yarn, the twist coefficient and the yarn structure of multi-filament yarn (untextured FDY or textured yarn) on yarn properties of ring core-spun yarn were investigated. For this purpose cotton-PET multi-filament ring core-spun yarn (Ne16) produced with four different core-sheath ratios, three different twist coefficients. For determine the effect of filament count of multi-filament yarn on yarn properties, two different filament counts were selected, 36 and 72. The ring core-spun yarns were produced at same condition with 100% PET untextured fully drawn yarns (FDY) and textured yarns. The effect of core-sheath ratio on the yarn tenacity values and the yarn elongation were found statistically significant. The yarn tenacity value of core-spun yarn increased with increasing the sheath ratio in yarn. The effect of twist coefficient on the yarn elongation was found statistically significant for only FDY multi-filament core material type. The yarn elongation (%) values increased with increasing the twist coefficient As the twist amount increased, it's seen that the yarn evenness and yarn hairiness values have been decreased. It has seen that the yarn evenness value decreased with increasing the core ratio in yarn, for both two core material types.