The purpose of this research was to study the dyeing of eri silk yarn using soaked red bean water. The optimal condition for preparing dye solutions was a ratio of 1:5 of dried red bean to water and dyeing at 100°C for 45 min. The dyeing process was conducted with and without mordants, using three different mordanting methods; pre-mordanting, meta-mordanting, and post-mordanting. The effects on silk analyzed are color fastness to washing, wet and dry rubbing, acid and alkali perspiration, and color characteristics on CIE L*a*b* color coordinates. Without mordants, the dyed silk yarn gave a rosy brown color. Each mordant caused shade changes on dyed yarns and mordant techniques affected the color intensity and color fastness of dyed eri silk yarn.