Abstract: Based on the precipitation of CaCO3 by adding sodium carbonate solution gradually to a solution containing calcium ion,a new and rapid method of photometric titration was proposed for the evaluation of efficiency of scale inhibitors used in the cycling water cooling system.An experimental installation was designed and fabricated for the determination with a spectrophotomer as the detector.A simulated hard water of 1.4 g·L-1 CaCl2 solution was used as test solution,and a definite amount of the solution was transferred to the absorption cell in which the tritration was performed.A definite amount of the scale inhibitor was added to the test solution,and during the process of titration with 1.4 g·L-1 Na2CO3 solution,a sudden drop of transmittance was observed due to the formation of CaCO3 precipitate in the solution.The volume of Na2CO3 solution consumed in the titration was a measure for the efficiency of scale inhibitor tested.More the volume of Na2CO3 solution consumed,better will be the efficiency of the inhibitor.