Low-dimensional　ZnO　nanocrystals　with　controlled　size,　aspect　ratio,　and　oxygen　defects　(e.g.,　type　and　concentration)　are　successfully　prepared　through　simple　solvothermal　and　thermal　treatment　methods.　The　structure　of　the　as-synthesized　samples　is　characterized　by　XRD,　N-2　physical　adsorption,　TEM,　and　IR　and　XPS　spectra.　The　results　show　that　the　aspect　ratio　and　size　of　the　as-synthesized　ZnO　nanocrystals　increase　with　increasing　[OH-]/[Zn2+];　the　morphology　evolves　from　nanorod　to　nanoparticle　with　an　increase　in　the　annealing　temperature;　the　BET　surface　areas　of　the　corresponding　samples　decrease　during　these　processes,　respectively;　and　different　oxygen　defects,　which　are　likely　to　be　oxygen　vacancy　(V-o(center　dot　center　dot))　and　interstitial　oxygen　(O-i　＇＇),　are　formed　in　our　experiments　accordingly.　With　evolution　of　the　structure,　IR　absorption　bands　and　visible　photoluminescence　emission　peaks　of　the　synthesized　ZnO　nanocrystals　shift　and　split,　which　is　ascribed　to　the　change　of　oxygen　defects.　In　addition,　it　is　found　that　the　photocatalytic　activity　of　the　synthesized　ZnO　nanocrystals　is　mainly　dependent　on　the　type　and　concentration　of　oxygen　defects.　The　relationship　of　structure-property　and　the　possible　photocatalytic　mechanism　are　discussed　in　detail.