The influence of Sn-substitution on structure dielectric properties and conductivity of CaCu3Ti4O12 ceramics was investigated Single-phase structures with space group Im (3) over bar were obtained up to x = 0 1 in CaCu3(Ti1-xSnx)(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 CaCu3(Ti1-xSnx)(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 10 kHz) XPS analysis results confirmed the obvious decrease of Ti3+/Ti4+ and Cu+/Cu2+ ratio and the decreased giant dielectric response was primarily attributed to the modification of the mixed-valent structure of Ti3+/Ti4+ and Cu+/Cu2+ (c) 2010 Elsevier B V All rights reserved