Minute boehmite crystals with high aspect rations, which were hydrothermally synthesized from gibbsite in $Ba(OH)_2$ solution, occluded Ba with the Ba/Al molar ratio of about 0.03 in their interlayers. Their surface areas were about 14$m^2$/g. The Ba-intercalated bohemite samples were partly used for producing $BaAl_{12}O){19}$ with low sinterability by externally supplementing $Ba(OH)_2$, and for forming transient aluminas. The surface area of $BaAl_{12}O){19}$ obtained by firing at $1500^{circ}C$ for 3 h was 5.3$m^2$/g, which was significantly lower than 12$m^2$/g of the sol-gel origin. While a mixture ${gamma}$-alumina and BaO is known to from $BaAl_{12}O){19}$ at $1200^{circ}C$, solid state reaction between η-alumina transformed from the Ba-intercalated boehmite and BaO formed from $Ba(OH)_2$ deposited on the boehmite started above $1300^{circ}C$. This suggests that large sized $Ba^{2+}$ ion occluded in η-alumina considerably suppresses the diffusion of $Al^{3+}$ ion. The surface area of the Ba-intercalated boehmite fired at $1400^{circ}C$ for 3h was as high as 14$m^2$/g indicative of its potential applicability to combustion catalysts. But it was decreased to 5.0$m^2$/g after firing at $1500^{circ}C$ for 3 h, accompanied by abrupt formations of $alpha$-alumina and $BaAl_{12}O){19}$ as main products. The suppression of $alpha$-alumina formation up to $1400^{circ}C$ also suggests the significant blocking effect of $Ba^{2+}$ ion on the diffusion of the component ions.