Microwave dielectric ceramics of (1 - x)Ba(Mg1/3Nb2/3)O-3-xBaSnO(3) [(1 - x)BMN-xBS] with high quality factors was synthesized by the solid-state reaction method. The effects of BaSnO3 additions (x = 0-0.2) on the sinterability, crystal structures, microwave dielectric properties, and microwave dielectric loss mechanisms of BMN were investigated systematically. The degree of 1:2 cation ordering was decreased with increasing Sn content and eventually faded away as x >= 0.1, where the low-temperature relaxations disappeared coincidently through the thermally stimulated depolarization current technique. It was supposed to be the short-range misplacements of the B-site cations within the long-range ordered structure. Meanwhile, the high-temperature relaxations associated with the in-grain oxygen vacancies were found in all the title compounds. Though the concentrations of oxygen vacancies of 0.8BMN-0.2BS were higher than BMN, high Q x f values could also be obtained even in the absence of 1:2 cation ordering. Specifically, the excellent characteristics like epsilon(r) = 29.02, Q x f = 90 000 GHz and tau(f) = 6.3 ppm/degrees C were achieved in the specimens of x = 0.2 sintered at 1450 degrees C.