The effects of Al2O3 content on the densification, phase compositions, microstructure and microwave dielectric properties of CaMgSi2O6-Al2O3 ceramics were investigated. The Al2O3 addition in CaMgSi2O6 ceramics resulted in the presence of a new phase, e.g. Ca(Mg,Al)(Si,Al)(2)O-6, and caused the emergence of Ca vacancies and O vacancies. The solubility limit of Al2O3 in Ca(Mg,Al)(Si,Al)(2)O-6 was found to be 4-8 wt%, and second phases of CaAl2Si2O8 and (Mg,Al)(8)(Si,Al)(6)O-20 were observed when the Al2O3 addition exceeded 8 wt%. Appropriate Al2O3 addition promoted the sintering process and lowered the densification temperature due to the vacancies defects caused by the Al3+ substitution for Mg2+ and Si4+ and the liquid phase originated from the second phases. Typically, good microwave dielectric properties of epsilon(r) = 7.99, Q x f = 60,132 GHz and tau(f)=-48.21 ppm/degrees C were obtained when the Al2O3 addition was 12 wt% and the as-prepared composite ceramic was sintered at 1250 degrees C. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.