Biocompatible Mg1− xZnxFe2O4 (x = 0.2, 0.4, 0.5, 0.6 & 0.8) nanoparticles were synthesized by solvothermal reflux method. All compounds were crystallized in cubic spinel structure with slightly enhance of lattice parameter with biocompatible substituent Zn2+ concentration. All compounds were shown spherical geometry with average particle diameter is around 12 nm (colloidal size). The spinel structure formation was confirmed by X-ray diffraction,electron diffraction, infrared, Raman shift measurements. Infrared analysis shows oleic acid coating on the surface of nanoparticles and TGA analysis shows that oleic acid desorbs from nanoparticle by decomposition at around 400 °C. UV-Vis-NIR spectra show all the compounds show energy band gap in the semiconductor range (≈ 1.9 eV). All compounds show superparamagnetic characteristics at room temperature with enhanced saturated mass magnetization (Ms) with Zn2+ concentration up to x = 0.5 and then reduces with further enhance of x up to 0.8. The Ms changes were ascribed to occupation of Zn2 + at tetrahedral sites and proportional enhance of Fe3+ at octahedral sites. The enhanced Fe3+ concentration at octahedral sublattice leads to formation Fe3+-O2−-Fe3+ networks which favor antiferromagnetic interactions due to superexchange phenomenon. Photocatalytic activity of all compounds were studied through methylene blue (MB) degradation analysis. All compounds show ≈ 96% degradation of MB upon 70 min irradiation of light on photoreactor vessel. In addition, photocatalytic activity (degradation efficiency) enhances with Zn2 + concentration in MgFe2O4. The Zn2+ substitution enhances both Ms and photocatalytic activity biocompatible of MgFe2O4 nanoparticles. © 2017