Mn-Zn ferrite, Mn1− x Zn x Fe2O4 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 (M s) of 7 nm particles was maximum for the x = 0.2 sample. AC magnetic susceptibility measurements were performed for Mn0.8 Zn 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. An AC field was found to cause the increase in temperature, with the 18 nm particles exhibiting the highest temperature increase, as expected. In addition, in vitro experiments were carried out to study the hyperthermia effects of Mn1− x Zn x Fe2O4 (x = 0.2, 18 nm) particles on human cancer cells.