The mechanomyography (MMG) records and quantifies the low-frequency lateral oscillations of active muscle fibers. It can represent the mechanical characteristics of muscle activity. MMG has been used to evaluate muscle fatigue. Hilbert-Huang transform (HHT) is a time-frequency method with the feature of self-adaptation, and designed specifically for nonlinear and nonstationary signal analysis. In this study, MMG signal was recorded from biceps brachii during isometric fatigue contraction. HHT was used to calculate the difference between the maximum and minimum values of instantaneous frequency, named as the band ratio, to estimate muscle fatigue. The results showed that the band ratios were 0.431 +/- 0.607 and 0.286 +/- 0.218 after fatigue for the maximum voluntary contraction (MVC) of 50% and 70%, respectively. These indicated that the frequency declined after muscles fatigue.