This paper presents a 2D tissue temperature estimation method for in-vitro monitoring a bovine liver using a diagnostic ultrasound when the tissue is heated under RF thermal therapy. The physical theory of the ultrasound temperature estimations is based on the linear relationship between the degree of temperature change and the ultrasound echo-shift. The RF echo signals were used to calculate the 2D phase changes and estimate the temperature distributions map. It is demonstrated that the ultrasound temperature estimation method can successfully estimate the 2D temperature distribution in bovine liver tissue for temperature ranging from 30°Cto 70°C. The temperature distributions map can also be shown on a regular B-mode ultrasound image by overlaying. This temperature estimation method shows the dynamics of the temperature distribution inside tissue, this information are very important for thermal dosage control and real time temperature monitoring during RF thermal therapy.