We have proposed magnetic hyperthermia treatment using our magnetic nanoparticles. Hyperthermia treatment is one of cancer therapy using heat effect. Mn-Zn ferrite, Mn_1-xZn_xFe₂O₄ nanoparticles encapsulated in amorphous SiO₂ were prepared using our original wet chemical method. X-ray diffraction patterns confirmed that the diameters of these particles were within 7 - 30 nm. Magnetization measurements for various sample compositions revealed that the saturation magnetization (Ms) of 7 nm particles was maximum for the x = 0.2 sample. AC magnetic susceptibility measurements were performed for Mn_0.8Zn_0.2Fe₂O₄ (x = 0.2) samples with 13 - 30 nm particles. The peak of the imaginary part of the magnetic susceptibility χ” shifted to higher temperatures as the particle size increased. From the result of AC magnetic measurements, sample of 18 nm would exhibit highest temperature increase. Measurement system of heating samples by AC field was constructed and observed temperature increase. An AC field was found to cause the increase in highest temperature with the 18 nm particles as expected. Finally, in vitro experiments were carried out to study the hyperthermia effects of Mn_1-xZn_xFe₂O₄ (x = 0.2, 18 nm) particles on human cancer cells.