The appropriate value of Manning’s roughness coefficient (n) is chosen through the process of calibration;i.e., the value which reproduces observed data to an acceptable accuracy. In the present study, the HEC-RASunsteady flow model is applied to Hilla river (upstream Hilla city) to predict the value of Manning’scoefficient through the calibration procedure. The data are taken for the period from 20 August 2008 to 12September 2008 and divided equally into two sets; the first set is for calibration purpose; i.e., estimation of(n) and the rest for verification which is the process of testing the model with actual data to establish itspredictive accuracy. It is found that the value of Manning's roughness coefficient (n) for Hilla river whichshows good agreement between observed and computed hydrographs is (0.027).