In　this　study,　a　microwave-induced　ablation　zone　(thermal　lesion)　monitoring　method　based　on　ultrasound　echo　decorrelation　imaging　was　proposed.　A　total　of　15　cases　of　ex　vivo　porcine　liver　microwave　ablation　(MWA)　experiments　were　carried　out.　Ultrasound　radiofrequency　(RF)　signals　at　different　times　during　MWA　were　acquired　using　a　commercial　clinical　ultrasound　scanner　with　a　7.5-MHz　linear-array　transducer.　Instantaneous　and　cumulative　echo　decorrelation　images　of　two　adjacent　frames　of　RF　data　were　calculated.　Polynomial　approximation　images　were　obtained　on　the　basis　of　the　thresholded　cumulative　echo　decorrelation　images.　Experimental　results　showed　that　the　instantaneous　echo　decorrelation　images　outperformed　conventional　B-mode　images　in　monitoring　microwave-induced　thermal　lesions.　Using　gross　pathology　measurements　as　the　reference　standard,　the　estimation　of　thermal　lesions　using　the　polynomial　approximation　images　yielded　an　average　accuracy　of　88.60%.　We　concluded　that　instantaneous　ultrasound　echo　decorrelation　imaging　is　capable　of　monitoring　the　change　of　thermal　lesions　during　MWA,　and　cumulative　ultrasound　echo　decorrelation　imaging　and　polynomial　approximation　imaging　are　feasible　for　quantitatively　depicting　thermal　lesions.