Abstract: The paper presents a method for on-line measurement of low-level γ radioactivity in large-volume effleunt water. The detection effeciency for liquid source of ~(40)K and point sources (~(60)Co, ~(137)Cs) are determined. Feasibility study shows that a volume about 1 m~3 of monitoring pool for φ 100×100 mm NaI (T1) detector is acceptable. The minimum specific activity that can be detected would be better than 9.2×10~2 Bq/m~3. It looks necessary to bubble out the radon gas in industrial. effleunt before the measurement.