In order to investigate the light fastness of Korean knotweed extract, the concentrate were prepared and dyed to silk fabric under the various conditions, such as kinds of mordants, methods of mordanting, and the dye concentration. After the dyed fabrics were irradiated for several hours, the color differences and K/S values were measured. Additionally, the light fastness of Korean knotweed extracts and Emodin in liquid solution was compared.The light fastness of silk fabrics dyed with Korean knotweed extract was improved by the introduction of Fe-mordant, but the surface colors on the dyed fabrics shaded into reddish and dark yellow by 80hours irradiation. Color difference of fabrics were increased with concentrations of Korean knotweed extracts. So inconsistent fading behaviour was observed. Color difference of dyed fabrics increased remarkably at the early stage of fading and became slowly down except for Fe-mordanted fabric. Absorbance of Korean knotweed extracts and Emodin in liquid solution were extremely decreased with concentrations of colorants, so inconsistent fading behaviour was observed too.