High-Intensity focused ultrasound (HIFU) is a type of therapeutic treatment in which ultrasound is focused to a target tissue such as cancer to be thermally coagulated. To enhance the safety and accuracy of HIFU treatment, a noninvasive method of monitoring the thermal lesion formation is important. A potential method for this purpose is ultrasound imaging. In this study, high-speed imaging by parallel beamforming was performed using ultrasound RF signals acquired during HIFU exposure, and the distribution of the cross-correlation coefficient between RF frames was calculated to estimate the tissue coagulation. Using high-speed imaging, HIFU can be irradiated quasi-continuously. The result shows that the decorrelation was observed at and around the focal spot of HIFU exposure. The decorrelation was induced by the change in RF signals owing to tissue coagulation, which was confirmed by the fact that the emergence and increase in the number of decorrelated pixels at and around the focal spot corresponded to the start and progress of tissue coagulation.