The internal resonance of electrostatically coupled cubic nonlinear low-frequency micro-resonator and linear high-frequency micro-resonator is investigated experimentally. A phenomenon of frequency locking is observed at 1:3 frequency ratio of two resonators for the first time when the coupling strength exceeds a certain value. Then the theoretical study is made to explain the frequency locking phenomenon. Finally, this phenomenon is used to enhance the short-time frequency stability of low-frequency resonator nearly 7-fold.