The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO3-doped Ba(Mg1/3Ta2/3)O-3 (BMT) system as a typical example of Ba(B'B-1/3"(2/3))O-3-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO3-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W6+ for Ta5+ in the WO3-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W-Ta(.) sites with a concomitant generation of tantalum vacancies (V-Ta''''') and mobile oxygen vacancies (V-O(..)) for the ionic charge compensation.