The Louis scheme and the COARE algorithm (version 3.0) are tested against eddy covariance and inertial dissipation methods for fric-tion velocity estimates in different wind-sea/swell regimes. Atmospheric forcing data, tabulated by Donelan et al. (1997.J Phys O-ceanog, 27: 2 087~2 099), were collected from a mast on the foredeck of a SWATH (small water-plane area, twin hull) ship in deep seaoff the State of Virginia during the surface wave dynamics experiment. These data are representative of low to moderate wind regimes.The aerodynamic roughness length is determined by using the Charnock relationship. The intercomparison shows that the Louisscheme and the COARE algorithm underestimate the friction velocity by 6% and 3% respectively under pure wind sea conditions, 15%and 13% respectively under cross swell conditions, and 21% and 17% respectively under counter swell conditions. The analysis showsthat these underestimations were caused by the method chosen to determine the aerodynamic roughness length because it significantlyunderestimates the aerodynamic roughness length. It is especially true under the cross swell and counter swell conditions. The Louis scheme and the COARE algorithm (version 3.0) are tested against eddy covariance and inertial dissipation methods for fric-tion velocity estimates in different wind-sea / swell regimes. Atmospheric forcing data, tabulated by Donelan et al. (1997. J Phys O-ceanog, 27: 2 087-2 099), were collected from a mast on the foredeck of a SWATH (small water-plane area, twin hull) ship in deep seaoff the State of Virginia during the surface wave dynamics experiment. These The intercomparison shows that the Louisscheme and the COARE algorithm underestimate the velocity of the friction by 6% and 3% respectively under pure wind sea conditions, 15 % and 13% respectively under cross swell conditions, and 21% and 17% respectively under counter swell conditions. The analysis showsthat these underestimations were caused by the method chosen to determine the aerodynamic ro ughness length because it significantlyunderestimates the aerodynamic roughness length. It is especially true under the cross swell and counter swell conditions.