The theoretical formula of three stratified interfacial long waves are deduced and its validity is verified by the experiment. The three stratified interfacial long waves consist of two modes that are Mode I and Mode II. These waves velocity are expressed cI and cII in equation (1') respectively. The wave numbers of these waves are expressed k1 and k2 in equation (3). These stratified interfacial wave profiles of first and second interface are expressed ζI1, ζI2, ζII1 and ζII2 in equation (8) respectively. The water particle velocities of these waves in first, second and third stratified layers are expressed uI1, uI2, uI3, uII1, uII2 and uII3 in equation (9) respectively. These relations between the profiles and the velocities are shown in figure 2.
