Clothing acts as a second human skin and as a substitute for some biological properties that protect other animals from harsh weather conditions. With the help of clothes, humans are even able to enter very difficult conditions, such as space between planets. Over thousands of years, the physiological functions of clothing have steadily improved. In this regard, one of the most important functions of clothing is to act as an intermediary between the skin and its surroundings, to transfer moisture and heat produced by the body. In this project, the effect of the cross-sectional area of the fibers on the thermal properties is investigated. Three types of polyester fibers were used; one with a simple circular cross-section, one with a croection with four channels and one with six channels. The fabrics were produced in three different type of design i.e. Interlock two-cylinder texture designs and one regular rib twocylinder texture design. Interlock tissue designs include normal interlock, Half Milano interlock, and Half Cardigan interlock. Thermal properties such as thermal conductivity and thermal resistance coefficient of these fabrics were investigated one by one. The temperature of the cold side (outside) was recorded and examined with an infrared thermal camera. Physical properties of the samples such as porosity, weight per unit area and thickness of the samples were measured. Significance of cross-sectional area of polyester fibers was 1/114, so cross-sectional area of polyester fibers had an effect on heat transfer of the samples. Fiber with a circular cross-section has the highest heat conductivity due to its compact structure with low porosity. Fiber with a four-channel cross section is in second place, and fibers with a six-channel cross-section have the lowest heat transfer rate. The significance level of the weave pattern of the samples was 1/111, so the weave pattern of the samples affects the amount of heat transfer of the samples. Samples with rib weave pattern had the highest rate of heat transfer and samples with Half Cardigan interlock weave pattern due to the presence of Half-textured ring in their structure, trapped air better and more and had the lowest amount of heat transfer. The level of significance of the interaction of weave pattern -cross-sectional was 1/147, so the interaction of weave pattern -cross-sectional design on heat transfer rate is significant. The heat transfer rate of the sample with circular cross section and rib weave pattern was 39/28 and had the highest heat transfer rate. The heat transfer rate of the sample with six-channel cross section and Half Cardigan interlock weave pattern was 37/68 and had the lowest heat transfer rate