Abstarct: t In this article, the nonlinear vibrations of a simply supported beam to which a bistable NES (nonlinear energy sink) is locally attached, have been investigated. Bistable NESs are widely used due to their efficiency in comparison with mono-stable NESs. In such problems, the forced internal resonances at which the amount of energy exerted to the NES from the beam is large enough is of a great deal of interest. Hence, after obtaining and dimensionless the equations of motion of the system, Galerkin’s method has been implemented to discretize the equations of motion. Using the method of multiple scales (MSM), the 3:1 primary internal resonance has been investigated. The results show that the frequency response curves are highly influenced by slight NES parameter changes, especially by damping. However, due to the behavior of the NES in the frequency domain which unlike the beam is softening, restrictions imposed by physical limitations due to the NES mass displacement can negatively affect its efficiency. Also, continuous wavelet transform is applied to the dimensionless kinetic energy of the beam to assess the transient energy transfer in the beam in time domain and enhancing the targeted energy transfer in the entire system. Moreover, bifurcations and Poincaré sections of the system are used to analyze the regime of motion of the beam model for the aim of the NES design optimization at the resonance case.