A field experiment was conducted for collecting three-dimensional wind, wave, and air-sea environmental data off the coast of Louisiana in the Gulf of Mexico in the mixed sea state of a wind sea limited by fetch and with swell propagating against the wind direction. Two methods, the inertial dissipation (ID) and eddy correlation (EC), are used to calculate wind stress. The results show that under the fetch-limited condition, the EC wind stress is greater than the ID stress. This difference is found to be correlated with the counter-wind swell. The swell-related drag coefficient is computed from the wind stress difference between the ID and EC methods. An empirical formula is constructed for the swell-related drag coefficient with a regression coefficient of 0.71. The swell-related drag coefficient is proportional to the swell steepness (kpHs) and inversely proportional to the ratio of the wind speed and the swell peak phase speed (U10/Cp).