The effect of Sn4+ (BaSnO3) B-site substitution on the microstructure and dielectric properties of Ba(Mg1/3Ta2/3)O3microwave ceramics was investigated. X-ray diffraction shows that a complex perovskite material Ba(Mg1/3Ta2/3)O3was prepared. As Sn4+ content x increases in the (1-x) Ba(Mg1/3Ta2/3)O3-xBaSnO3 (x=0.00～0.20) system, the dielectric constant generally keeps unchanged, while TCF changes from positive to negative. Although the addition of Sn4+ reduces the ordering degree, Qf0 is still increased when the ceramics density increases. This trend implies that Qf0 of this system is mostly determined by ceramics density rather than ordering degree. After sintering at 1500℃for 3 h, the system with x=0.15 was found to have excellent dielectric properties as follows: ε≈25, Qf0≥300,000 GHz at 7 GHz, TCF=-0.6×10-6/℃.