Crystal structure and microwave dielectric properties of Ba(Mg1/3Nb2/3) $O_3$ (BMN) ceramics were investigated. Ba(Mg1/3Nb2/3) $O_3$ has the 1:2 ordered hexagonal structure. The 1:2 ordering and relative density of specimens increased with small Mg deficiency(x). The variation of Q${ imes}$ $f_{0}$ with Mg deficiency is very similar to that of 1:2 ordering and relative density. The highest Q${ imes}$ $f_{0}$ achieved in this investigation is about 96,000 for Ba(Mg1/3Nb2/3) $O_3$. The improvement of Q${ imes}$ $f_{0}$ with Mg-deficiency is related to the increase of degree of ordering and relative density of the specimen.men.