The natural dyeing of silk fabric with Rheum undulatum L. extract was investigated. The proper colorant concentration, dyeing temperature, dyeing time, and pH for the dyeing of silk fabric with Rheum undulatum L. extract were 90% v/v, $90^{circ}C$, 80 minutes and pH 3, respectively. In various mordanted methods, the K/S values of mordanted methods were higher than those of unmordanted methods. Light colorfastness of mordanted silk fabric was improved by mordanting. The antibacterial properties of dyed and mordanted silk fabric showed a high reduction rate, and Cu mordant was showed the most effective bacterial reduction.