In synthetic/spandex blends fibers, contents of the synthetic fiber by percentage is higher than those of spandex. Thus, the dyeing process of the blends fiber is mainly focused on the synthetic fiber, but not the spandex. In this study, we used several series of vat dyes for dyeing the nylon/spandex blend fabrics and their dyeing properties(fk value) were investigated at various reduction agents, temperatures, pH which have the potential for deep shade dyeing effects. The formamidinesulfinic acid were found to be effective to increase the fk values of the blend fiber compared to sodium hydrosulfite and Rongalit C. Higher temperature($100^{circ}C$) and lower pH(pH 4) were observed as suitable conditions for maximum fk values.