Spinel ferrite nanoparticles are widely studied to evaluate their suitability in many applications. Magnetic, electrical, optical, and catalytic properties of spinel ferrites nanoparticles depend on: i) intrinsic property such as cation distribution among tetrahedral (A-site) and octahedral ligand (B-site) sublattices in spinel structure and ii) extrinsic properties such as specific surface area and particle size distribution of nanoparticles. To enhance the above properties by modifying both the intrinsic and extrinsic parameters, narrow size distributed Mg0.7Zn0.3Fe2O4 mixed spinel ferrite nanoparticles are synthesized by the solvothermal reflux method using high boiling point reflux solvents. The particles are crystallized in a cubic spinel structure and are single crystallites with an average particle diameter of 12 nm, measured from an electron micrograph. The temperature-dependent dielectric constant (varepsilon{prime}) and dielectric loss (varepsilon{prime prime}) of the sample show no change below 290 °C and increase with temperature up to 450 °C. Both varepsilon{prime} and varepsilon{prime} decrease with the increase of electric field frequency and shows dominant space charge polarization at grain boundaries. The DC conductivity estimated from impedance spectra (Cole-Cole plot) shows the Arrhenius model electron hopping conductivity mechanism above 390 °C. Nanoparticles show high Ms (57.41 eμg) than that synthesized by other methods. The sample shows the magnetic hyperthermia value of 189 W/g at 1 mg/mL concentration. The sample degraded 95% of rhodamine B dye in water in 320 min under UV light illumination. Some of these properties are superior to Mg0.7Zn0.3Fe2O4 nanoparticles synthesized by other wet chemical and/or ball milling methods. © 1965-2012 IEEE.