The hydrothermal reactions of fibrous H2Ti4O9 particles with Ba(OH)(2) solution in the presence of cationic surfactants of n-hexadecyltrimethylammonium hydroxide (HTMA-OH) and n-hexadecyltrimethylammonium bromide (HTMA-Br) were investigated in a temperature range of 150-250 degrees C. H2Ti4O9 phase with layered structure was transformed to BaTiO3 phase in the Ba(OH)(2)-(HTMA-OH) and the Ba(OH)(2)-(HTMA-Br) solutions, and partially transformed to anatase phase in the Ba2+-free HTMA-OH and HTMA-Br solutions by topotactic structural transformation reactions under the hydrothermal conditions. The cocoon-like BaTiO3 and titanium oxide particles were obtained after the hydrothermal reactions in the Ba(OH)(2)-(HTMA-OH) and HTMA-OH solution, respectively. These cocoon-like particles were formed by assembling the fibrous particles in the surfactant solutions.