Manning's roughness coefficient for flood plains with macro roughness elements is experimentally investigated in an 18 m long and 1 m wide laboratory channel. Velocity field was measured in the main and flood plain channels. Total discharge was obtained both from weir measurement and from integration of measured velocity field. Four different surface patterns including smooth and various arrangements of macro roughness elements were considered. Prismatic concrete volumes were placed as macro roughness elements on the flood plains. Wall shear stresses were determined from measured velocity profiles using logarithmic wall functions. Energy slope was determined from the total tractive force obtained by integration of wall shear stresses. Finally, Manning's roughness coefficient was determined from the Manning's formula applied to main channel and the flood plain channels. It is found that the roughness coefficient on the flood plain channels may increase more then two folds when about 2% of the flow volume in the flood plain is occupied by the macro roughness elements.