In ultrasound thermotherapy (USTT) high intensity focused ultrasound (HIFU) is used for noninvasive thermal treatment of human tissue deep inside the body. In this paper a FDTD-model is presented to simulate USTT. It combines nonlinear ultrasound propagation and broadband tissue attenuation together with the bio-heat transfer equation for calculation of temperature distribution in tissue. The temperature dependence of parameters is integrated in the complete model. Simulation results demonstrate the potentialities of this simulation tool to analyze and optimize thermotherapy.