A simple, low-cost, and non-contact optical method using modified Michelson interferometry is proposed for measuring the vibration frequencies of objects. Two beams which are split from a laser beam project onto two rough surfaces (compared with the laser-beam wavelength) of a fixed reference object and a vibrating tested object respectively, and two quasi-spherical waves scattered or diffused from the two rough surfaces individually are combined to form interference fringes on an observation plane. The vibration frequencies of the tested object can be derived by analyzing the time-dependent gray-value signal (caused by the fringe-shifting phenomena) for a specified position on the observation plane. The signal is transformed from the time domain to the frequency domain to form a spectrum by first using the fast Fourier transformation. Then, all the spectrum values corresponding to different frequencies in the spectrum are compared with a threshold value. Finally, all frequencies in the spectrum with local peaks higher than the threshold value are considered to be the vibration frequencies of the tested object. The experiment is a non-contact type with no lenses and is able to be conducted easily with high accuracy and low cost for testing objects vibrating with one, two, or three frequencies. The results and features confirm the utility of the proposed method.