At present time the understanding of the physical nature of two kinds of compressional waves in saturated soils with extreme permeability is still not clear. For example the velocities of two compressional waves in case of infinite permeability given in different literatures are inconsistent. By using the dynamic basic formulations of saturated soils given by Zienkiewicz

the establishment of pore water motion equation is discussed. Then one-dimensional compression controlling equations and wave equations in u-w and u-W forms are derived

from which the compressional wave velocities in extreme cases of zero and infinite permeability are obtained; and their physical natures are also explained. The conception of inertial-coupling force is put forward; and it is emphasized that in dynamic analysis of saturated soils the interaction between soil skeleton and pore water consists of two parts: seepage force and inertial-coupling force. The parameter of measuring inertial-coupling-force is porosity; and its influence on compressional wave velocities is discussed.