The invention relates to a use of a stem cell conditioned medium to inhibit melanin formation for skin whitening. First, mesenchymal stem cells are cultured in a cell culture dish containing complete growth media. After mesenchymal stem cells are sub-cultured in the basal media for three times, a conditioned medium can be acquired from the basal media. Accordingly, cosmetics containing at least 2.5 wt.% conditioned medium can effectively suppress tyrosinase activity and inhibit melanin formation so as to achieve efficacy of skin whitening.
