The improved dielectric properties and voltage-current nonlinearity of nickel-doped CaCu3Ti4O12 (CCNTO) ceramics prepared by solid-state reaction were investigated. The approach of A-site Ni doping resulted in improved dielectric properties in the CaCu3Ti4O12 (CCTO) system, with a dielectric constant epsilon approximate to 1.51x10(5) and dielectric loss tan approximate to 0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X-ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co-existence of Cu+/Cu2+ and Ti3+/Ti4+. A steady increase in epsilon(f) and a slight increase in observed upon initial Ni doping were ascribed to a more Cu-rich phase in the intergranular phase caused by the Ni substitution in the grains. The low-frequency relaxation leading to a distinct enhancement in epsilon(f) beginning with CCNTO25 was confirmed to be a Maxwell-Wagner-type relaxation strongly affected by the Ni-related phase with the formation of a core-shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current-voltage characteristics of the CCTO ceramics. The present A-site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.