The indophenolblue colorimetric method combined with a microplate reader with standard 96well microplate format was developed to determine ammonium in water samples.The detection limit was 0.046 mg/L.Values of recovery were in the range of 90.7%~101.8% when adding the standard NH+4N solution to five water samples.The NH+4N values measured by the colorimetric method with microplate reader(X）were correlated with by continuous flow analysis and salicylic acid spectrophotometry（Y).The regression equation was Y=1.052 4X0.009.The correlation coefficient was 0.976 1**(n=32，P<0.01）and linear slope was near 1.0.The relative standard deviation(RSD）of proposed colorimetric method with microplate reader was less than 3%（n=6).It is suggested that the indophenol blue microcolorimetric assays based on a standard 96well microplate format and microplate reader could be used to simplify the quantification of ammonium in water samples.