The causes of yellowing occurring in the heat treatment of knit fabrics made of cotton/spandex 2-ply yarn were investigated. Extracts of the spandex yarn and the cotton yarn separated from the knit fabric which had been preset, scoured and heat set were obtained using $CCl_4$ and analyzed using UV-Vis spectrophotometer, FTIR, and GPC. The results indicate that the spinning oil used in spandex yarn and polyurethane oligomers were contained not only in the spandex yarn but also in the cotton yarn. Most of the spinning oil and some polyurethane oligomers in spandex yarn transferred to cotton yarn. Scouring did not completely remove then and about 40% remained in cotton after scouring. The color of spandex extract heat-treated at 16$0^{circ}C$ was yellow but the color of pure spinning oil did not change with the same heat treatment, which suggests yellowing is caused by polyurethane oligomers. The yellowing occurs mainly in the cotton yarn rather than spandex yarn, as the yellowing occurs due to the thermal decomposition of polyurethane oligomers which have been transferred to cotton yarn along with the spinning oil.