Abstract: In this paper a multipath problem in shallow water acoustic channel and phase conjugation technology will be presented. Acoustic communications in shallow water had been a difficult problem due to the characteristics of the underwater acoustic channel. The shallow water channel is the environment which is of particular interest to many research workers and presents a number of challenges to the designers of communications systems. Underwater acoustic channels are characterized by multipath phenomenon whose characteristics are time varying. This is the major factor that limits underwater acoustic communications performance. Communication systems can be categorized as using either incoherent or coherent modulation strategies. This paper presents the phase conjugation technique, also called “time-reversal”, which is the subject of active research in many areas of acoustics. This is a new method for coherent underwater acoustic communication. Preliminary results confirm the potential utility of the phase conjugation in underwater acoustics with applications in underwater communications. This method effectively mitigates the multipath propagation and can be used to solve this problem.