In this research, a feasibility study to measure a magnitude and a cycle of a time-varying stress of a specimen by a natural frequency was carried out. An experiment was conducted. We used a specimen of a round bar with 8 mm diameter and 290 mm span length which was fixed at both ends. A sinusoidal axial stress was applied to the bar. A deflection of the bar in a free vibration was measured by a laser beam displacement device. To collect much information on the deflection, a device which hit the bar periodically was made up. A fast Fourier transform method, a short-time Fourier transform method and a wavelet analysis were applied to the deflection. The methods give us relations among time, a frequency and a magnitude of a signal with complicated representations. Applying the analyses to the experimental data, we tried to evaluate a magnitude and a cycle of a time-varying load.