In this study, MnxFe2-xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) nanoparticles were synthesized by polyol route and the effect of Mn3+ substitution on structural and dielectric properties of Fe3O4 was studied. X-ray powder diffraction (XRD) patterns confirmed the single spinel ferrite phase formation (Rietveld analysis). Crystallite size of the synthesized materials lie in the range of 12-25 nm as calculated X-ray diffraction patterns using Scherrer's formula. The microstructural features were examined by SEM images. Cation distribution calculations confirmed Fe3+ ions have both tetrahedral and ochedral site preferences whereas Mn3+ ions mostly occupies tetrahedral A-site. The ac electrical and dielectric properties of Mn3+ ion substituted Fe3O4 nanoparticle show that there were significant changes in both conductivity and complex permittivity as well as dielectric loss tangent as Mn3+ ion concentration is varied from zero to unity. It is clearly seen that conductivity increases with increase in temperature which may be due to increase in hopping capability of charge carriers at higher temperatures. Detailed evaluation of analysis reveals that at higher frequencies there is less effect of temperature on conductivity which can be interpreted on the basis of interfacial, dipolar, ionic and electronic polarization as detailed above section in explanation of variation of dielectric loss. The dielectric permittivity shows dielectric behavior can be clarified on the basis of Koop's interpretations in accordance with two layer Maxwell-Wagner model by accounting for surface charges. The electrical and dielectric properties, i.e. ac/dc conductivity, real/complex dielectric permittivity, and dielectric loss (tan S) decrease with Mn3+ ion doping in some aspects. (C) 2015 Elsevier Ltd. All rights reserved.