We investigated the characteristics of nonlinear acoustic resonant frequency shift and three-wave mixing interaction during tensile test for low-carbon steel of S25C with electromagnetic acoustic resonance (EMAR). EMAR is the application of a resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk- shear-wave EMAT, which transmits and receives shear wave propagating in thickness direction of a plate specimen. Nonlinear acoustic parameter variation has indicated incremental behavior in relation to the plastic strain, and reached to the peak value between lower yield strength strain and tensile strength strain. Those characteristics are corresponding to the dislocation densities measured by X-ray.