Cotton fabrics were dyed using the extract from red onion skins. The differences between red and brown onion skin and the differences between cotton and silk were investigated. The colors of the fabrics dyed with and without six types of mordants, including Mg2+, Al3+, Ca2+, Ti4+, Fe3+ and Cu2+, were measured by a spectrophotometer and expressed according to the CIELAB color system and ΔE* value. The red onion skin contained quercetin, which is also found in brown onion skin, in addition to malvidin-3-arabinoside of anthocyanins. The cotton was dyed a light color at a temperature of more than 80℃. The color of the dyed cotton did not change after an immersion period longer than 8 h. No significant damage or changes in the structure of the fibers as a result of the dyeing were detected. The color fastness to light and to washing (color change) of the fabrics was enhanced by mordant treatment of some metal ions.