Bach Dang river located within the area of 106°37'00" - 107°00'00" E and 21°00'00"- 20°35'00" N, runs through two districts of Hung Yen (Quang Ninh) and Thuy Nguyen (Hai Phong). Along its banks are concentrated with important industrial zones of Hai Phong and big seaports with busy shipping. These activities are mainly reasons impacting on water quality in the area. This paper presents the change of amoni concentration in Bach Dang estuary based on monitoring data in July 2008, March 2009 and monthly. monitoring from October 2009 to October 2010. Monitoring results show that .water in Bach Dang estuary is polluted by ainoni. Amoni concentration ranged from 16.55 to 562.3pg/l, exceeded Vietnamese standard (QCVN 10:2008). Amoni concentration is usually high from July to September and low from Octerber to December. The stratification (high value in surface layer) of amoni concentration is obviously observed in the dry season. Along transects, the tendency of amoni concentration is decreasing from near-shore stations to offshore stations. This indicates that amoni is mainly from landbased sources.