Magnetic properties through controlled nucleation and crystallization wer studied for ferrimagnetic 40Fe2O3-20CaO-40SiO2 glass useful as thermoseeds for hyperthermia of tumor therapy. The maximum nucleation and crystal growth temperature are $700^{circ}C$ and 100$0^{circ}C$ respectively. The glass showed the maximum saturation magnetization of 168.4 emu/cm3 when nucleated $700^{circ}C$ for 60 min and crystal grown at 100$0^{circ}C$ for 4hrs. The maximum coercive force was 390 Oe when uncleated $700^{circ}C$ for 60 min and crystal grown at 975$^{circ}C$ for 2 hrs. The variation of the saturation magnetization could be well quantitatively interpreted in terms of the volume fraction of the magnetite whereas that of the coercive forces could be explained only qual.itatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 0.18W/cm3 when heat treated at 100$0^{circ}C$ for 4 hrs pre-necleated at $700^{circ}C$ for 60 min and temperature increase of 7K under AC magnetic field of 10 kOe and 10kHz.