Abstract: we prepared low dielectric constant microwave dielectric ceramics(system of K₂O-Al₂O₃-SiO₂)doped MgO in the way of solid state reaction, discussed the effect of MgO on the system of K₂O-Al₂O₃-SiO₂ about phase structure、micro-structure、Sintering temperature and Dielectric properties by SEM, XRD and Agilent4284.The result show that MgO could lower sintering temperature, and the samples are low dielectric constant and low dielectric loss at room temperature. At the frequency of 1 MHz, the sample doped with 2(wt)%MgO has the lowest dielectric constant sintered at 1 150 ℃, and the lowest dielectric loss sintered at 1 250 ℃.