Abstract: The first-order small slope approximation is applied to the problem of the sound transmission from an airborne source into deep ocean through a rough sea surface, and expressions are derived for the transmitted sound field and its coherent component. Numerical calculations are performed. The sea surface is assumed to be random rough with a Pierson-Moskowitz spectrum and have height variations in only one dimension. For the case of the airborne line source, the small slope approximation results are in good agreement with those from integral equations, and show that the mean of sound intensity at observation direction with shallow depression angle increases and approaches a limit as the root-mean-square surface height increases, while the coherent field intensity consistently decreases. For the case of the point source, the small slope approximation results show that the mean of sound intensity depends significantly on the source-receiver bearing angle, but the coherent field intensity is independent of this angle.