Setting behavior of spandex yarn in heat treatments for the anti-crease treatment of knit fabric made of cotton/spandex 2-ply yarn was investigated. Three different methods, i.e., dry-heat treatment, hot water treatment, and steam treatment, were used to set the spandex yarns. Setting properties were measured via length changes of the treated samples. Tensile strength retention and elongation of the treated samples were also measured. The results indicate that length changes of the samples treated with hot water and steam are larger than that treated with dry heat due to the presence of moisture. The length change of spandex yarns increases with increasing treatment temperature, time and extension. Among the three factors, extension has the most significant effect on the setting behavior. Spandex yarn set at a certain condition can be set again if it is treated under more severe conditions. Tensile strength and elongation of spandex yarn decrease with increasing length change.