The bankfull discharge has high correlation with the hydraulic geometries of width and depth by regime theory analysis. The averaged velocity of bankfull discharge is correlated greatly with channel slope but little with the roughness of bottom substrates in a streamflow. The equation for bankfull discharge in gravel bed channel developed by Parker(2006), which is characterized by the depth to d50 of bottom substrate in addition to the hydraulic geometries, should be modified such that the roughness term of the depth to d50 is substituted by the bankfull width to depth of hydraulic geometry. The estimated bankfull discharge by the developed equation had great coincidence with measured one by bankfull indicators of hydraulic geometry with the high RMSE of 15.51 m³/s and MAPE of 9.93 m³/s.