Cotton fibre properties show differences according to their region of cultivation, which are caused by the use of various kinds of cottonseeds as well as differing climate conditions. Therefore, the blending and yarn spinning processes of cotton fibres from crops harvested in different regions should be carried out under control, and the blend ratios should be selected in order to ensure the optimum yarn quality properties. Otherwise, the variations existing in fibre properties will also cause variations in yarn properties. In Turkey some South-East Anatolian (GAP) cotton fibres are transported to the Aegean region and are blended with cotton fibres from this region. Such a situation causes some problems for textile manufacturers. In this study, the differences between the properties of Aegean and GAP cotton fibres and the changes in yarn quality as an effect of blending these two sorts have been estimated As a result, the quality characteristics of produced yarns such as the structural properties (yarn count, yarn twist), the unevenness properties (CV% values, thin and thick places, neps, hairiness) and the physico-mechanical properties (tenacity and elongation at break) have been determined, and the results obtained statistically evaluated. We wish to draw this study to the attention of producers and textile manufacturers.