The primary purpose is to do cancer risk assessment of toxaphene by using four steps of risk assessment proposed by the United States National Academy of Sciences/National Research Council (NAS/NRC). Four steps of risk assessment including hazard identification, dose-response relationship, exposure assessment, and risk characterization were used to evaluate cancer risk of toxaphene. Toxaphene was the most heavily used insecticide in many parts of the world before it was banned in 1982. It increased incidence of neoplasms of liver and uterus in mice and increased incidence of neoplasms of endocrine organs, thyroid, pituitary, adrenal, mammary glands, and reproductive systems in rats. From mice's and rats' study, slope factor for toxaphene is 0.8557 (mg/kg/day)^-1. Lifetime average daily dose (LADD) of toxaphene from ambient air, surface water, soil, and fish were 1.08 × 10^-6, 5.71 × 10^-6, 3.43 × 10^-7, and 7.96 × 10^-5 mg/kg/day, respectively. Cancer risk of toxaphene for average exposure is 7.42 × 10^-5. From this study, toxaphene might have carcinogenic risk among humans.