The　effect　of　post-treatment　HCI　concentration　on　titanate-titania　transformation　was　investigated.　Titanate　was　prepared　by　the　alkaline　hydrothermal　treatment　of　TiO2,　and　then　transformed　to　titania/titanate　upon　subsequent　washing　with　0.10-0.55　mol.L-1　HCI.　The　titanate　almost　completely　transformed　to　rutile　when　treated　with　HCI　concentrations＞0.78　mol.L-1.　Accompanying　this　phase　transformation,　the　morphology　of　the　sample　changed　from　nanotubes　to　nanosheets　and　nanoparticles.　This　was　related　to　the　rate　of　phase　transformation,　which　depended　on　the　HCI　concentration.　Titanate　became　resolved　into　detached　TiO6　octahedra　during　HCI　treatment,　and　titanate-titania　transformation　occurred　via　rearrangement　of　the　TiO6　octahedra.　The　phase　transformation　and　morphological　evolution　were　studied　by　X-ray　diffraction,　and　transmission　and　scanning　electron　microscopies.　The　linear　and　nonlinear　optical　properties　of　the　products　were　investigated　using　ultraviolet-visible　absorption　spectroscopy　and　the　open　aperture　Z-scan　technique,　respectively.　The　minimum　transmittance　at　Z=0　and　maximum　optical　limiting　effect　were　obtained　when　the　HCI　concentration　was　0.55　mol.L-1.　Different　nonlinear　optical　effects　were　exhibited　by　different　morphologies.