Abstract: Received ultrasonic signals from long bone contain multiple superposed guided modes. In this paper, Hilbert- Huang transform (HHT) was applied to analyze multi-mode guided waves and decompose waves to many independent modes. Group velocity of independent mode was obtained at corresponding frequency and compared with the results with short time Fourier transform (STFT). Cortical thickness was also obtained by comparing with the theoretical results. The results showed that the experimental determined thickness was in agreement with the actual cortical thickness. It is indicated that measuring the velocity of the guided mode can be used to estimate the cortical thickness. Also, the HHT is an effective method to identify multimode guided waves.