This paper presents a computer simulation model to study the bio-mimetic fish modeled as a compliant beam. The aim is to determine the natural frequencies of the fish and then compute and characterize the mode shapes and relate them to propulsive thrust. The application of model is focused on design of propulsion systems with compliant bodies for bio-mimetic locomotion in marine environments. The compliant bodies are designed for their natural frequency modes (i.e. The mode that most resembles the desired kinematics) and since they operate in the matching range of natural frequencies they are energy efficient. In the model fish is assumed as a bio-mimetic compliant beam of a varying cross sectional parameters and 'Timoshenko Beam (TB)' theory is used in the computer simulation model to compute mode shape and thrust. The presented results clearly establish the mode shape and the resultant thrust. © 2015 MTS.