Activated sludge process is considered one of the most common and highly effective methods used in aerobically biological treatment systems. The design of such systems is usually based on the biological kinetic approach considerations. The present study is concerned in determining the biological kinetic of the last part of Diyala River at AL-Rustimiyah WWTP's, Baghdad, Iraq. A completely mixed continuous flow lab-scale reactor without recycling was used for this purpose. Various detention times were adopted during the experimental work ranging from 0.723 to 3.809 days. Influent and effluent BOD5, MLVSS and MLSS for the aeration tank, among other tests were performed at different detention times, after reaching the steady state conditions, in order to generate the required data for bio-kinetic coefficients. The biological kinetics k, Y, Kd, and Ks for the last part of Diyala River at AL-Rustimiyah WWTP's were found to be 5.68 d-1, 0.75, 0.06 d-1, and 70 mg/l, respectively. These values were compared with the bio-kinetics of different types of wastes and were found to be within the typical ranges of bio-kinetic parameters for activated sludge process treating domestic wastewater, which indicates that the water at the river reach of interest is rather wastewater than pure river water.