Effect of light and heat resisting additives on fading of a acid dye(C.I. Acid Red 361) on nylon 6 BCF and degradation of the fiber were studied. These additives were doped in the fiber as a master batch chip during spinning process to improve dye fading or fiber degradation when nylon 6 BCF was exposed under UV-irradiation and heat. The dye fading was affected by not only UV-radiation but also heat especially above $100^{circ}C$. With applied additives the dye fading caused by UV-radiation or heat could be lowered but the effect was not remarkable. The tensile strength retention of undyed sample was higher than that of dyed sample when the fiber was exposed under UV-irradiation. Thus it is concluded that the acid dye creates active radicals more easily than fiber during exposure of UV-irradiation and acts as a photo accelerator in degradation of nylon 6 BCF. On the other hand, the tensile strength retention of undyed sample was lower than that of dyed sample when heat was applied to the fiber. This could be attributed that radicals created during fiber degradation by heat preferably attack dye molecules rather than transfer to nylon 6 fiber molecules which causes chain scissoring. Light and thermal degradation of nylon 6 BCF were greatly improved by doping with a UV-absorber, light resisting agents or heat resisting agents to the nylon 6 BCF, but it is not easy to improve dye fading. Thus new combinations of light and thermal resisting agents showing more effective to both thermal and light fading of dye are necessary for higher light fast dyeing of nylon 6 BCF.