Underwear is laundered frequently and most of them is made of cotton, but a cotton fiber is more difficult to modify than a synthetic fiber. We have attempted to determine the optimum conditions necessary whereby the lowest concentration of silver solution is needed to produce the greatest antimicrobial properties of cotton fabrics. For this study, colloidal silver was made by electrolysis. The concentration of colloidal silver was increased by increasing the area of the silver plates submerged in the water, the water temperature, the water hardness and the flow time of the water per 1l. However, the colloidal silver concentration was decreased by extending a space between the silver plates and increasing the water velocity. Cotton fabrics treated in the washing machine with 1.3 ppm colloidal silver solution for 10 minutes had effective microbial properties and an unperceivable reduction of reflectance