Cotton fibers are the most important raw material of the textile industry. Clean collection of cotton fibers increases the quality of textile products and the market prices. This study was conducted to investigate the effects of normal and late harvest period of different cotton varieties on cotton fiber and yarn properties. The quality properties of the fibers were determined with the HVI device, and the quality properties of the yarns obtained from the fibers were determined with the USTER TESTER device. The fiber quality properties such as fiber fineness (micronaire), fiber maturity, fiber length (UHML, mm), strength (g/tex), brightness/whiteness (Rd), and yarn quality properties such as yarn evenness (%), thin places, neps+200 (%) and hairiness were investigated in the study. The exposure of fibers to rain during the harvest period of 2018 caused a decrease in the quality properties. Inability of the plants to receive sufficient sunlight during the last physiological development period caused a decrease in photosynthesis rate. This negative situation negatively affected the physical quality properties of fiber and yarn. The results indicated that the physical quality properties of fibers and yarns obtained in 2019 were higher than those in 2018. The aim of this study was to determine the effect of late harvest period on fiber and yarn quality properties and thus on quality of textile products. In addition, the study was carried out to guide researchers who will conduct researchers on similar topics.