Abstract: A flow-injection analyzer was used for the determination of total phosphorus in water. The water sample was digested with H2SO4 and potassium persulfate at 125 ℃. Phosphosrus, present as ortho phosphate in the sample solution was reacted with ammonium molybdate to form heteropoly acid the absorbance of which after reduced by ascorbic acid was measured photometrically. Linear relationship between values of absorbance and mass concentration of total phosphorus was obtained in the range within 1.0 mg·L-1, with values of detection limit (3S/N) of 0.003 mg·L-1 and lower limit of determination (10S/N) of 0.012 mg·L-1. The proposed method was applied to the determination of total phosphorus in water samples, values of recovery found by standard addition method were in the range of 95.7%-102.5% with RSD′s (n=7) in the range of 1.5%-3.0%.