Using high intensity focused ultrasound (HIFU) to locally perform thermal ablation has shown its potential in auxiliary excision surgery, especially in thermal ablation of tumor. However, it is challenging for clinical physicians to locate the region being under HIFU thermal ablation and determine the applied ablation level only by conventional ultrasound B-mode images, causing dangers in therapy. In this study, we propose an ultrasonic Nakagami imaging technique to monitor HIFU thermal ablation in real time. It has been demonstrated that ultrasonic Nakagami imaging is capable of characterizing tissues with different scatterer concentration and distribution. Because in histological sections with and without HIFU thermal ablation, there are differences in scatterer concentration and distribution, we hypothesize that Nakagami parameters of tissues being HIFU thermal ablated will change, accordingly, thus potentially enabling Nakagami visualization of HIFU-induced thermal lesions. Ex vivo thermal ablation experiments of porcine livers were conducted to demonstrate our idea. Experiment results showed that Nakagami images could reveal the HIFU-induced thermal lesion, which was difficult to be located in conventional B-mode images because of no appearance of bubbles. Moreover, in the cases with apparent bubble formation, both of B-mode images and Nakagami images could locate the position of the thermal ablation area, while Nakagami images owned higher contrast. In summary, we experimentally demonstrated the feasibility of monitoring HIFU thermal ablation by Nakagami imaging. The contrast of the ablated regions between the Nakagam images before and after thermal ablation outperform that of B-mode images. Because the complexity of the Nakagami imaging algorithm is low, it can be easily integrated as part of post processing in current array systems indicating that real-time visualization of HIFU thermal lesion with Nakagami imaging is possible.