Variations in the high-frequency dielectric, thermal and mechanical properties of cyclo-olefin polymer (COP)/MgO composites with MgO content were investigated. Dielectric properties were measured using a circular cavity resonance method at approximately 12 GHz and dielectric constants (epsilon(r)) were found to increase with increasing MgO concentration. The variations in the observed epsilon(r) values were in good agreement with Bruggeman mixing theory, and the dielectric loss (tan delta) values of the composites were consistently in the order of 10(-4). It was confirmed that exposure to humid conditions had little effect on the dielectric properties of the composites. The temperature coefficient of the dielectric constant (TCe) and the coefficient of thermal expansion (CTE) were improved as the MgO fraction was increased due to the superior thermal properties of MgO. The mechanical properties of the composites were assessed by Charpy impact and flexural tests. The flexural moduli of the composites increased with higher MgO concentrations in accordance with Nielsen's theory, while deteriorations in the flexural and impact strengths were observed.