Co2+, Ni2+ and Co2+/Ni2+ co-doped beta-MnO2 were synthesized by hydrothermal reaction to study the influence of crystal defects on the microwave dielectric response. The samples were characterized by X-ray powder diffraction (XRD), X-ray fluorescence spectrometry (XRF), transmission electron microscopy (TEM), and vector network analyzer. The construction of defective beta-MnO2, with doping of Co2+ and Mn/O vacancies, was also discussed based on first-principles calculation. Results showed that the crystallographic defects played important role in modifying the microwave dielectric performance. The dielectric loss capacity at 2-18 GHz was enhanced after doping of Ni2+ and Co2+ which was ascribed to the ionic polarization process. In the deficient configurations, the appearance of Mn vacancy narrowed the band gap, forming weak bound electrons and weak contact ions. Then the electronic and ionic relaxation polarization processes took place among these charged particles in the applied field. Mn vacancy was the most important factor in controlling the microwave polarization process, followed by O vacancy and extrinsic Co2+. (C) 2015 Elsevier B.V. All rights reserved.