Amorphization of two kinds of titanium oxide powders, anatase and rutile, during a planetary mill treatment was investigated. The amorphization of anatase markedly progresses despite its average particle size which is much smaller than that of the ground product. The powders are mechanochemically hydrated to form agglomerates in acetone with low polarity. The shear stress generated during mill treatment induces plastic deformation in the defloccuration process of the agglomerates, resulting in the promotion of amorphization. The addition of water to acetone accelerates the amorphization by promoting hydration. This amorphization process through the agglomeration-defloccuration process suggests a high possibility of synthesizing composite particles composing of hydrated oxide and inorganic salts with high polarity.