In this paper, we present the natural frequencies and mode shapes of the flexural vibration of different beam theories, Euler-Bernoulli beam theory (EBT) , Timoshenko beam theory (TBT) and Reddy beam theory (RBT), immersed in the viscous fluid. Furthermore, we research in different effects of natural frequencies between not only EBT and TBT but EBT and RBT due to different aspect ratio and dimensionless material properties, the G/E ratio. From the results, we conclude that aspect ratio, dimensionless material properties, and mode number play important roles in different beam models immersed in the viscous fluid. The dominant factor of beam structure immersed in the fluid with low G/ E ratio is fluid viscosity, whereas the fluid density with high G/E ratio. However, the result points out when the G/E ratio from 0.05 to 0.1, the influence of fluid viscosity and density will reduce to vanish. Accordingly, we should only consider the effect of aspect ratio and mode number.