Mn-Zn ferrites doped with different contents of Sm3+ and Gd3+ ions were prepared by sol-gel auto-combustion method and characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). When samples were calcined in a relatively low temperature below 1100 °C, secondary phases (α-Fe2O3) could be identified. Therefore, in order to acquire pure and better crystallinity, the suitable calcining temperature of powders was selected at 1200 °C. It was also found that all the samples consisting of ferrite phases of typical spinel cubic structure and average crystallite sizes between 31.5 and 38.2 nm were obtained after calcining at 1200 °C for 4 h. The lattice parameters increased almost linearly with increasing Sm content. A dense microstructure was obtained after sintering at 1250 °C for 4 h. Through the analysis of magnetic properties, hysteresis loops for all the samples were narrow with low values of coercivity and retentivity, indicating the paramagnetic nature of these samples. And saturation magnetization Ms strongly depended on the type of additive to reach a maximum of 47.99 emu/g for x=0.015, which showed a great promise for hyperthermia applications.