Rate of biodegradation of organic matter presented the capacity of self-purification in waters. The changes of the decay rate constants of organic matter would be able to impact significantly on approaches of water quality control. Based on laboratory experiment results of ten water samples collected at Be Estuarine (Khanh Hoa) and solved the Streeter & Phelps' function (1925) in the case of BOD, the decay rate constant of organic matter ranked in 0.0643 - 0.3202 /day. This constant was contributed by concentration of organic matter in water bodies, kind of organic matters, tidal systems and water exchanges. Thus, the suitable management of organic matter waste, which discharged directly in the waters, would help to improve the capacity of recovering waters.