Discrepancies between hyperecho-predicted necrosed volume in ultrasound (US) images and the actual size of a thermal lesion might cause incomplete ablation or damage normal structures during high intensity focused US (HIFU) ablations. A novel dual-frequency sonication procedure is proposed to reduce this discrepancy. HIFU transducers of either 1 or 3.5 MHz were applied to transparent tissue-mimicking phantoms and ex vivo bovine liver samples. A diagnostic probe and a charge-coupled device (CCD) camera were used to record lesion formation in real time, allowing for comparison of the sizes of the hyperechoes in US images and the protein denaturing area on optical images. Bovine liver specimens were segmented to reveal the lesion's terminal sizes. Differences between actual lesion volume and hyperechoes in US images were demonstrated to be dependent on acoustic frequency and intensity. At a low frequency (1 MHz), the hyperechoes appeared to be larger than the actual volume, but the difference decreased with the duration of ablation. In contrast, at a high frequency (3.5 MHz), the hyperechoes were smaller for ablations lasting longer than 10 s. Moreover, given certain low-intensity conditions, lesions were formed without detectable hyperechoes (3.5 MHz), or hyperechoes appeared before a visible lesion was formed (1 MHz). Dual frequency sonications (low frequency followed by high frequency) produce more stable and larger lesions, and with less position shift, which might be useful for designing future ablation strategies.