Onions are commonly available and easily processed, and since their skins are thrown away they could be very useful materials from the viewpoint of environmental preservation. This study aimed to process onion skins into the state of powder and look into the optimum condition for hair dyeing by decoloring virgin hair four times and observing the hair dyeing quality and its mechanical and morphologic changes by the different condition of onion skins in duration, temperature, density, pH, and mordant treatment. The observations of hair surface through scanning microscopy showed the formation of cuticle layer, though in a little blown-up state in the case of the hair dyed with onion skin application, when compared with bleaching hair with the completely dissolved cuticle layer. According to the above findings of experiments, the dyeing quality of the hair dyes with onion skin application was excellent, the dyeing was also feasible without mordants, and the use of Fe as the mordant increased dyeing exhaustion more, which would make it an effective hair dye. These results of the experiments indicate that the natural pigment extracted from onion skins can be actually used for hair dyeing from the viewpoints of the dyeing quality and the skin treatment.