The exchange of momentum between the oceans and atmosphere is important for many atmospheric and oceanic processes and is, mainly, governed by the roughness of sea surface. The latter can be expressed by a roughness length, zo. A roughness length model, based on the concept that zo is determined by stochastic wave breaking, is presented and evaluated. The model performance is tested using both measurements from the Östergarnsholm site in the Baltic Sea, taken under neutral stratification conditions, and information from other pertinent investigations. The wave field, and the roughness length variation, is investigated during various sea states dominated by wind-driven waves. The comparison of field and model results suggest that the present model can well reproduce the observed roughness length values and sea-drag coefficients, under the described sea state conditions.