Ferrimagnetic glass-ceramics are useful as thermoseeds for hyperthermal treatment of malignant tumors, especially those located in deep parts of the body, since hysteresis loss heating occurs locally at the cancer site when they are implanted around the tumor and subjected to an alternating magnetic field. In the present study, glass-ceramic powders containing a large amount of fine particles of zinc-iron ferrite (Zn_xFe_3-xO₄) in a CaO-SiO₂-based glassy matrix were prepared by the heat treatment of ZnO-Fe₂O₃-CaO-SiO₂ glasses at various temperatures for 1 h in a 95CO₂+5H₂ atmosphere. The Zn_xFe_3-xO₄ precipitated above 700°C, and its crystallite size and fraction increased with increasing heat-treatment temperature, reaching 130 nm and 37 mass% at 1150°C, respectively. The saturation magnetization of the specimen increased to 38.4 emu•g^-1 with increasing heat-treatment temperature up to 1150°C. The coercive force of the specimen increased to 167.6 Oe at 1000°C, and then decreased with increasing heat-treatment temperature. When the heat-treatment period at 1150°C in a 95CO₂+5H₂ atmosphere was increased from 1 to 5 h, the content of Zn_xFe_3-xO₄ was not changed, whereas the crystallite size of Zn_xFe_3-xO₄ increased from 130 to 170 nm. The saturation magnetization and coercive force of the powders heat treated at 1150°C for 5 h in a 95CO₂+5H₂ atmosphere were 26.5 emu•g^-1 and 150.5 Oe, respectively. The prepared glass-ceramic powders showed a large amount of heat generation of 12.4 W•g^-1, under conditions of 300 Oe and 100 kHz.