Phase composition, microstructure and tunable dielectric properties of (1 - x)BaZr0.25Ti0.75O3-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg2+ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below -100 degrees C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant alpha((T)) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr(0.25)Ti(0.75)O(3)-0.4MgO sample at 10 kHz and room temperature. (C) 2011 Elsevier Ltd. All rights reserved.