<p>A new method for mass detection based on the internal resonance of mode coupling in a piezoelectric microbeam resonator sensor is developed. By manipulating the actuation voltage, the 3:1 internal resonance between the first and second modes is triggered. The strain gradient theory is employed to characterize size effects. Governing equations are derived by using the Hamilton’s principle. Theoretical and numerical solutions are obtained by employing the multiscale method and the Runge–Kutta method, respectively. The results indicate that the amplitude-frequency response of internal resonance is related with the mass and adsorption position of a particle: The abrupt jumps in response amplitude between different steady states can facilitate the detection of minute particles. More specifically, the identification of the particle’s attachment position and mass is achieved through the shift in bifurcation point frequency.</p>