Microcystins　(MCs)　are　toxic　compounds　produced　by　cyanobacteria　in　eutrophicate　water　environment　and　threaten　the　drinking　water　quality　which　often　leads　to　serious　sicknesses.　MCs　are　difficult　to　be　removed　in　water　treatment　when　the　concentration　is　very　lowbut　still　harmful.　When　theMC　concentration　is　low　(mu　g/L),　filter　or　some　conventional　chemical　does　not　work,　but　UV　can　keep　removing　it　to　a　lower　level　by　some　active　groups.　Herein,　185-nmUVirradiation　in　an　immersing　mode　was　used　to　remove　MCs.　Compared　with　the　normal　radiation　mode,　the　immersing　mode　showed　a　remarkable　degradation　rate　ofMCs　and　a　greater　removal　efficiency　than　the　direct　radiation.　Radicals　of　center　dot　H　and　center　dot　OH　were　produced　and　strengthened　the　removal　rate,　after　H2O　absorbed　185　nm　photons.　Three　important　factors　of　pH　value,　initial　concentration,　and　aeration　capacity　were　investigated.　When　pH　was　less　than　7,　a　better　removal　rate　by　center　dot　H　was　found,　due　to　the　main　path　ofMC　degradation　and　Adda　strain　removal.　When　the　initial　concentration　increased,　the　MC　removal　ratio　decreased　because　HO　center　dot　formed　near　the　lamp　surface　and　degradedMCmolecules　fast.　When　the　aeration　capacity　improved,　theMCremoval　ratio　for　the　presence　of　air　enforced　reaction　of　dissolved　oxygen　with　hydrated　electrons　and　hydrogen　atoms　produced　in　the　radiolysis.