A composition comprising a pharmaceutically effective amount of zeaxanthin or its derivative for use in treating a malignant tumor and a method of using a pharmaceutically effective amount of zeaxanthin or its derivative either alone or together with one or more pharmaceutical agents for treating a malignant tumor. The tumor may be, but is not limited to breast cancer, cervix cancer, colon cancer, cutaneous melanoma, cutaneous squamous carcinoma, hepatocellular carcinoma, lung cancer, osteosarcoma, prostate cancer, and uveal melanoma. The pharmaceutically effective amount of zeaxanthin is generally above about 0.5 mg/kg/d to about 20 mg/kg/d.
