Dyeing on silk with vat dyes which shows good color fastness at strong alkaline conditions were newly examined. The optimum conditions of dyeing bath to prevent the damage on the tensile strength and elongation were sodium hydroxide(NaOH) 10g/l, sodium hydrosulfite(Na2S2O4) 20~22 g/l, for 30 min at 5$0^{circ}C$. The color yield of silk fabrics was increase according to the concentration of NaOH and Na2S2O4, but decreased at higher concentration than that of NaOH 10g/l and Na2S2O4 22g/l. The vat dyeings showed excellent color fastness against washing and light, compared with the conventional dyeings.