CaCu3Ti4O12 presents colossal dielectric permittivity within a large temperature and frequency range, which makes it to be a suitable material for technological applications, such as components of capacitive memories and mobile phones. In this investigation, SrTiO3-doped CaCu3Ti4O12 ceramics were prepared by solid-state reaction. The influence of doping on the structures, compositions and dielectric properties of the materials were investigated by X-ray diffraction, scanning electron microscopy and dielectric measurements between 40 Hz and 110 MHz. The material presents colossal response (εr~104−105) and the dielectric loss tangent decreased with doping level increase at high frequency. The microstructure analysis showed that the second-phase particles segregated in the doped CaCu3Ti4O12 grain edges. Cole-Cole modeling correlated well the effects of this segregation with the relaxation parameters obtained. The extrinsic contributions for the dielectric response were discussed together with the structural and compositional evolution of SrTiO3-doped CaCu3Ti4O12 material. The experimental results indicated that SrTiO3 doping is a suitable method to optimize the dielectric response and electrical properties of CaCu3Ti4O12 for the applications in microelectronic devices.