The present study is carried out to optimize the dyeing conditions for silk fabrics using a new natural dye, Vervain barks at different pH values in the absence of mordants. Also, experiments are implemented to investigate the effects of processing conditions on color difference, color scale, color strength and light fastness characteristics. The changes in the morphology of the silk fabric surface after dyeing are studied using a scanning electron microscope. The pH in the acidic range 1-5 is optimum for dyeing, showing very good fastness properties for silk dyed fabrics.