A study was conducted to investigate the effects of the flow velocity and the paving material on the nitrogen conversion rates in shallow open channels. Equal amounts of hollow holed and plain balls, all with a 3.5-cm diameter, was fastened to the channel bottom. Then the nitrogen conversion rates were taken at various velocities and compared. The inner and outer surfaces of the holed ball channel were about 1.5 times than of the plain ball channel, but the attached biofilm mass was found to be about 3.55 times greater. The optimal nitrogen conversion rates were found to be higher in the holed ball channel. A suitable velocity was determined which would enhance the optimal nitrogen conversion rates. The choice of velocity and the biofilm carrier to enhance or predict the nitrogen transformations in shallow channels, should be considered in model development.