The effect of Ba(Zn1/3Ta2/3)O3 (BZT) ceramic filler on the dielectric, mechanical and thermal properties of high density polyethylene (HDPE) matrix have been investigated. The dispersion of BZT particles in the matrix was varied up to 0.45 volume fraction (V f). The SEM images confirmed the increase in connectivity between the filler particles with the increase in filler loading. All the composites showed excellent densification (>99 %) with relatively low moisture absorption (<0.04 wt%). The dielectric properties of the composites were investigated at 1 MHz, 5 GHz and at 10 GHz. The relative permittivity and the dielectric loss were found to increase as a function of BZT loading. Different theoretical models were used to predict the relative permittivity at 10 GHz. Effective medium theory gave the best correlation with the experimental results. An enhancement in the thermal conductivity (TC) and a reduction in the coefficient of linear thermal expansion (CTE) were achieved with filler loading. A slight decrease in the tensile strength was also observed with BZT loading. At 10 GHz, 0.45 Vf BZT reinforced HDPE showed a low relative permittivity (εr = 8.2) and a low dielectric loss (tanδ = 1.6 × 10-3) with good thermal (TC = 1.4 W m-1 K -1, CTE = 92 ppm/ C) and mechanical (tensile strength = 18 MPa) properties. © 2013 Springer Science+Business Media New York.