Abstract: These studies were conducted to estimate nitrification in coastal water of Meizhou Bay by¹⁵N ammonium oxidation method. The nitrification rate of 0.51~4.60 μmol·L^-1·d^-1 were recorded, and the higher nitrification rate in water close to aquaculture area were propitious to ammonium self-purification. Higher nitrification rate was observed at high NO₃^- concentration water column, which showed NO₃^- produced mostly from nitrification. The biological turnover times for NH₄⁺ were short (1~9 d) in coastal water of Meizhou Bay with the longer turnover times in surface seawater. Nitrification transformed NH₄⁺ to NO₂^-, final to NO₃^-, reducing the harm to organisms of coastal aquaculture system resulted from NH₃-N and NH₄⁺-N.