Composites of ferroelectric BST and non-ferroelectric MgO are sintered for obtaining low dielectric constant and controlled microstructure of the two phases. Microwave sintering route was used for controlling the reactivity and the grain boundaries in the final sintered material. Ba1-XSr XTi:03:MgO composites were sintered in the microwave at a frequency of 2.45 GHz. The low dielectric constant & tangent loss and high tunability could be achieved in the resulted composites. With microwave sintering, the high relative density and controlled microstructure (and particle size) of BST:MgO composites could be achieved. In microwave sintered BST:MgO, minimum reactivity and high K-factor values are obtained.