Single-phase Ba2Ti9O20 materials, which are free from secondary phases have been successfully synthesized by using inverse micro-emulsion process. Although the BaTi5O11 phases were resulted preferentially when the as-prepared powders were calcined (800-1000 degreesC, 4h), they can be converted into Hollandite structure (Ba2Ti9O20) after sintering the samples at 1200-1300degreesC (4h). The prime factor facilitating the formation of Hollandite phases in micro-emulsion process is presumably the atomic scale mixing of cationic species with proper proportion. Dielectric constant measured at 7 GHz varies insignificantly with calcination and sintering conditions for the sample possessing a density higher than 94% T.D. In contrast, the Q x f value increases with the density of the samples provided that no abnormal grain growth phenomenon has occurred. The best microwave dielectric properties obtained are K = 39 and Q x f = 22,000, which corresponds to the 1250 degreesC x 4 h sintered samples prepared from 900 degreesC-calcined. (C) 2002 Elsevier Science B.V. All rights reserved.