Ceramic ferrites can be used to cancer-treatment. Heating of certain organs or tissue up to temperature between 42oC and 45oC preferentially for cancer therapy is called hyperthermia. We synthesized ferrites with various compositions in the system Co1-xNixFe2O4 as hyperthermic thermoseed in cancer-treatment and evaluated their effects on the necrosis of cancer cells under alternating magnetic field in vivo as well as in vitro. When a CoFe2O4 was placed into 0.2 ml distilled water, the greatest temperature change in this study, Δ T=29.3oC, was observed. More than half of the carcinoma cells were dead after exposure to alternating magnetic field using CoFe2O4, while normal cells were survived more than 60%. The injection of this ferrite particles into the tumor bearing mice was able to suppress the number and volume of tumors. CoFe2O4 is expected the useful hyperthermic thermoseed in cancer-treatment because it exhibited the greatest necrosis of carcinoma cells in vitro and in vivo.