The CoTmxFe2-xO4(x <= 0.1) NPs were synthesized sonochemically. X-ray powder diffraction patterns and TEM images of the samples prove their chemical purity and cubic structure-morphology, respectively. Some substitution ratio of thulium ions to cobalt ferrites have an important effect on the characteristic evaluation of both electrical and dielectric characteristic measured at frequencies up to 3.0 MHz between room temperature and 120 degrees C. Since the thulium substitution has a very strong effect on the characteristic evaluation of both electrical and dielectric properties of cobalt-ferrite samples, four substitutional ranges none, small, medium and high were determined for the interpretation of contribution of thulium ratio to ac/dc conductivity, dielectric constant, dielectric loss and tangent loss. Conductivity increases with the incremental frequencies, in general depending on a variety of tendencies of both temperature and substitutional Tm ratios while the activation energy varies with a high dependency to the regional level of Tm substitution in Co-ferrites NPs. The Arrhenius graph appears to provide us with a single activation energy much higher than 400 meV for x = 0.02, which can be attributed to electron hopping mechanisms, apart from other substituted spinel ferrites. (C) 2020 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.