To　study　the　effect　of　alkalinity　on　partial　nitrification　(PN)　and　the　feasibility　to　control　the　ratio　of　nitrite　to　ammonia　in　effluent,　the　PN　was　rapidly　started-up　in　a　sequencing　batch　reactor.　The　ammonia　conversion　rate,　ammonia　oxidation　rate　and　the　microbial　activity　were　investigated　in　different　ratio　of　the　inflow　alkalinity　to　ammonia.　Results　showed　that　the　PN　could　be　achieved　quickly　by　pre-cultivating　the　sludge　with　high　ammonia　nitrogen,　and　over　96%　nitrite　accumulation　rate　was　sustained.　The　rate　can　be　controlled　flexibly　with　the　liner　relationship　between　ammonia　conversion　and　the　ratio　of　alkalinity　to　ammonia　while　alkalinity　is　insufficient.　Cycling　experiments　show　a　good　indication　of　alkalinity　on　nitrite,　the　alkalinity　less　than　50　mg/L　will　stop　the　ammonia　oxidation,　and　the　inorganic　carbon　less　than　3.0　mmol·g-1　will　lead　to　the　reduction　of　microbial　population　and　activity.　In　actual　operation,　the　ratio　of　nitrite　to　ammonia　in　effluent　can　be　effectively　controlled　by　the　alkalinity.