The natural dyeing of silk fabric with Purple-Fleshed Sweet Potato (PSP) was investigated. The colorant was extracted with distilled water, and the color difference (${Delta}E^*$) was increased with increasing the amount of PSP in extraction. The proper temperature and time for the extracting of colorant with PSP were $60^{circ}C$ and 60 minutes. The optimum temperature, time and pH for the dyeing of silk with extracted PSP were $60^{circ}C$, 60 minutes and pH 4 respectively. In various mordanted methods, the color difference values of post-mordanted silk fabric were higher than those of pre- and simultaneous-mordanted method. And the wide range of colors( GY, Y, YR, R, RP) were obtained according to various mordants, mordanting methods and mordant concentrations. Light colorfastness of the mordanted silk fabric was improved. Laundering colorfastness, dry cleaning colorfastness and perspiration colorfastness were shown to be good.