This study focused on the effect of core- sheath ratio, twist and loop length on the thermal comfort properties of single jersey knitted fabrics produced from 100% cotton, 80:20 and 60:40 cotton/polyester core spun yarns. Thermal properties like air permeability, water vapor permeability, thermal conductivity and thermal resistance properties were analyzed with low, medium and high twist and also with loose medium and tight structured fabrics. Box behnken, three level - three variable factorial design software has been used to study the interactive effect of core- sheath ratio, twist and loop length on the thermal comfort properties of single jersey knitted fabrics and response surface equations were derived and design variables were optimized. ANOVA test model reveals the statistical significance of variables on thermal properties with high F-value and low p-values. From this study it is observed that the decrease in cotton ratio decreases the fabric thickness, becomes lighter and more porous with higher thermal conductivity, air permeability, water vapour transmission rate and less thermal resistance. Increase in twist and tight loop structure makes thicker and less porous fabric with higher thermal resistance and lesser air permeability, water vapour transmission rate and thermal conductivity.