The dielectric properties at microwave frequencies and the microstructures of nano (alpha + theta)-Al2O3 ceramics were investigated. Using the high-purity nano (alpha + theta)-Al2O3 powders can effectively increase the value of the quality factor and lower the sintering temperature of the ceramic samples. Grain growth can be limited with theta-phase Al2O3 addition and high-density alumina ceramics can be obtained with smaller grain size comparing to pure alpha-Al2O3. Relative density of sintered samples can be as high as 99.49% at 1400 degrees C for 8 h. The unloaded quality factors Q x f are strongly dependent on the sintering time. Further improvement of the Q x f value can be achieved by extending the sintering time to 8 h. A dielectric constant (Er) Of 10, a high Q x f value of 634,000 GHz (measured at 14 GHz) and a temperature coefficient of resonant frequency (tau(f)) of -39.88 ppm/degrees C were obtained for specimen sintered at 1400 degrees C for 8 h. Sintered ceramic samples were also characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). (C) 2007 Elsevier Ltd. All rights reserved.