Abstract: Ultrasonic backscatter signals from the cancellous bone can be used to diagnose osteoporosis effectively due to its ability to provide the information of bone microstructure.Mean trabecular bone spacing(MTBS) is one of important parameters for characterization of bone microstructure.This paper proposed a MTBS estimating method based on the fundamental frequency estimation,which was applied to backscatter signals from simulations,and bovine trabeculae in vitro.The estimated MTBS results were compared with those of simplified inverse filter tracking(SIFT) algorithm and the autoregressive(AR) cepstrum method.The results demonstrated that the proposed method is very robust for the MTBS estimating with more precise results and smaller estimating variation in the presence of a small signal-to-noise ratio,and a large scattering strength ratio of diffuse scatterers to regular ones.