Cobalt-doped CCTO (CaCu3Ti4O12) ceramics were prepared by a conventional sol–gel synthesis method and the effects of Cobalt doping on the microstructures and dielectrical properties were investigated. The phase composition and microstructure were studied by means of X-ray diffraction (XRD) and high resolution scanning electron microscopy (HRSEM). Efficient crystalline phase formation accompanied by Cobalt induced lattice constant expansion was confirmed through XRD studies. HRSEM results show that doping effectively enhanced grain growth or densification. A compositional study reveals the variation of Cobalt diffusion in CCTO structure by the reduction of Ti presence according to the doping ratio. The dielectric constant reached a value as high as (εr = 25,400 at 1,000 and εr = 111,000 at 1,050 °C) at a cobalt-doping concentration of x = 0.2 at low frequency (50 Hz). The dielectrical constant and dielectric loss of the pure and cobalt-doped CCTO ceramics was measured for different concentrations and discussed in detail.