Abstract: A new noninvasive temperature estimation method is developed based on the ultrasound backscattering spectrum, in which biotissue is supposed to be inhomogeneous continuum as ultrasound is radiated in it. The fluctuations of ultrasound speed and tissue density cause the scattered wave. The average power spectrum of scattered wave is dependent on the ultrasound speed c, the ultrasound speed gradient with respect to temperature dc/dt alld the average temperature of tissue T. Using the relationship of the ultrasound speed to temperature in tissue, temperature information in tissue can be extracted from the average ultrasound power spectrum of scattered wave. Theoretical analysis and experimental result in this paper provide a new way to estimate tissue temperature in hyperthermia.