Zn-Ni ferrite nanoparticles were successfully synthesized by coprecipitation method. The samples were annealed at various temperatures, i.e., 200 °C, 400 °C, 600 °C, 800 °C, and 1000 °C. The nanoparticles have the mixed spinel phase structure as confirmed by the X-ray diffraction patterns. The crystallite size was 15.1 nm and increased to 25.1 nm after annealing at 1000 °C. Transmission electron microscope images showed that the annealed sample exhibited better dispersion and grain boundaries compared to the as-prepared sample. Fourier transform infra-red spectra showed the existence of vibrations at 378 cm−1 and 555 cm−1, confirming bonding for mixed spinel ferrites. The hysteresis measurement by using vibrating sample magnetometer confirmed that the sample possessed soft magnetic properties with a coercivity of 45 Oe and increased after annealing. The saturation magnetization of the as-prepared sample was 11 emu g−1, and increased to 58 emu g−1 after annealing at a temperature of 800 and 1000 °C. The specific absorption rate (SAR) with an alternating current magnetic field (50 Hz and 100 Oe) of Zn-Ni ferrite before and after annealing (at 800 °C) was 63.7 and 92.4 mW g−1, respectively. The results showed that annealing temperature has a significant role in determining the microstructural, the magnetic properties and the SAR of the nanoparticles.