Roughness Coefficient (RC) is one of the important and effective factors in open channel design. The precision value RC is necessary to design and utilize earth canals. Therefore, the current study was conducted to evaluate roughness coefficients in vegetated channels by Cynodon dactylon at the Moghan plain (in North-west of Iran). In the network of Morgan, 30 drainage canal sections were selected to measure water flow velocity (with a flow meter) and canal cross sections (with profilimetery devices). Results revealed that RC varied from 0.01 to 0.4 and averaged as 0.08 in vegetated canals by Cynodon dactylon. Also, flow velocity, flow section area, wetted perimeter, hydraulic radius and roughness coefficient were lognormal in distributions. Results also showed that flows were turbulent and with an increase in Reynolds numbers, roughness coefficients decrease. Sensitivity analysis of flow rate to roughness coefficient showed that with increase as 200 and 300 percent in roughness coefficients, flow rates were 0.50 and 0.33 of flow rate from average roughness coefficient. Also, with decrease 50 and 25 percent in roughness coefficients, flow rates were 2 and 4 folds of flow rate from average roughness coefficient. Thus, sensitivity of flow rate to roughness coefficient was significant and it is recommended necessary precision in the engineering estimation and computations in the selection of roughness coefficients.