The influence of Sn-substitution on structure, dielectric properties and conductivity of CaCu 3 Ti 4 O 12 ceramics was investigated. Single-phase structures with space group Im3¯ were obtained up to x=0.1 in CaCu 3 (Ti 1−x Sn x ) 4 O 12 . The giant dielectric constant step became lower with Sn-substitution by depressing both the low and high temperature dielectric relaxations. The room temperature dielectric constant and dielectric loss of CaCu 3 (Ti 1−x Sn x ) 4 O 12 decreased with increasing Sn-substitution. The dielectric constant decreased from 79,846 to 31,324 when x changed from 0 to 0.1, while and the dielectric loss was suppressed from 0.083 to 0.062 (at 10kHz). XPS analysis results confirmed the obvious decrease of Ti 3+ /Ti 4+ and Cu + /Cu 2+ ratio, and the decreased giant dielectric response was primarily attributed to the modification of the mixed-valent structure of Ti 3+ /Ti 4+ and Cu + /Cu 2+ .