The　objective　of　this　work　was　to　understand　and　optimize　the　flocculation　of　a　marine　alga　Nannochloropsis　oculata　with　two　cationic　salts,　aluminum　sulfate　(AS),　and　ferric　chloride　(FC).　Based　on　single-factor　and　response-surface-methodology　experiments,　second-order　polynomial　models　were　developed　to　examine　the　effect　of　initial　algal　biomass　concentration　(IABC),　pH,　and　flocculant　dose　(FD)　on　final　solid　concentration　of　algae　(SCA).　The　experimental　and　modeling　results　showed　that　SCA　favored　low　pH,　which　however　was　undesirable　to　biomass　recovery　rate.　There　existed　a　positive　stoichiometric　relationship　between　FD　and　IABC;　higher　IABC　required　higher　FD,　and　vice　versa,　for　higher　SCA.　Optimum　flocculation　conditions　were　predicted　at　IABC　of　1.7　g/l,　pH8.3,　and　FD　of　383.5　μM　for　AS,　and　IABC　of　2.2　g/l,　pH7.9,　and　FD　of　438.1　μM　for　FC,　under　which　the　predicted　maximum　SCA　were　32.98　and　30.10　g/l　using　AS　and　FC,　respectively.　The　predictions　were　close　to　validation　experimental　results,　indicating　that　the　models　can　be　used　to　guide　and　optimize　the　flocculation　of　N.　oculata　using　AS　and　FC　as　the　flocculants.©　Springer　Science+Business　Media　New　York　2013.