The dielectric relaxation and ac conduction of CaCu3Ti4O12 (CCTO) ceramics were investigated at different temperatures under a dc bias. The dc biases give rise to the space charges accumulating, namely electrode response, resulting in the significant increase of dielectric permittivity and dielectric loss tangent. Two Debey-like relaxations, arising from both electrode and grain boundary response, present in low frequency with the increase of the dc biases. The electrode relaxation and grain boundary relaxation are distinguished according to the impedance spectroscopy and the frequency dependent ac conductivity. The relaxation time of electrode and grain boundary is 0.955 ms and 0.026 ms with a dc bias of 10 V at 328 K, respectively.