Abstract: Proceeding from the dispersion equation of the normal mode in homogeneous shallow water, we derive several formulae calculating the mode attenuation and group velocity, and compare their accuracy by means of numerical results. This paper especially discusses the attenuation and the group velocity of the critical mode in a Peketis channel and shows that, in general, the attenuation and the group velocity of the critical mode respectively are less than the absorption and the sound speed in the bottom. Therefore, the Kornhauser, Reney, Weston and Tindle's conclusions are modified that the attenuation and the group velocity of the critical mode are exactly equal to the absorption and the sound speed in the bottom, respectively.