An investigation is made of the sound attenuation in a rectangular duct with two sides lined and with a uniform steady flow. It is found in the study of modal attenuation that the fundamental mode is not necessarily the least attenuated in lined ducts. It is shown that sound attenuation is influenced by the fluid flow; tuning frequency shifts to higher frequency with decreasing peak attenuation for downstream propagation and to lower frequency with increasing peak attenuation for upstream propagation. The effect of acoustic impedance on sound attenuation for a given duct geometry is presented as well as the effect of duct geometry on sound attenuation for a given acoustic impedance. Theoretically predicted attenuation spectra are compared with experimental results (test data for both rectangular flow duct and Boeing 747/JT9D engine). The predicted attenuation spectra are in very good agreement with the experimental results for downstream propagation. However, the present theory overpredicts the sound attenuation for upstream propagation.