Mechanical changes in breast tissues as a result of cancer are usually detected through palpation by the physician and/or self examination. However, physicians are unable to palpate most masses under 1 cm in diameter and microscopic diseases. The goal of our study is to introduce the application of the Harmonic Motion Imaging (HMI), an acoustic radiation force technique, for reliable sensitive tumor detection and real-time monitoring of tumor ablation. Here, we applied the HMI technique using a single-element Focused Ultrasound (FUS) transducer. Due to the highly localized and harmonic nature of the response, the motion characteristics can be directly linked to the regional tissue modulus. In this experiment, a confocal transducer, combining a 4.68 MHz therapy (FUS) and a 7.5 MHz diagnostic (pulse-echo) probe, was used. The FUS beam was further modulated by a low AM continuous wave at 25 Hz. A pulser/receiver was used to drive the pulse-echo transducer at a Pulse Repetition Frequency (PRF) of 5.4 kHz. The radio-frequency (RF) signals were acquired using a standard pulseecho technique. The intensity amplitudes of the FUS beam at the focus (Ispta) were 231 W/cm2 for tumor detection and 1086 W/cm2 for FUS ablation. An analog bandpass filter was used to remove the spectrum of the FUS beam prior to displacement estimation. The resulting axial tissue displacement (i.e., HMI displacement) was estimated using an RF-based speckle tracking technique based on 1D cross-correlation. For tumor mapping, a harmonic radiation force was applied using a 2D raster-scan technique. The 3D HMI image was obtained by combining multiple 2D planes at different depths. The 2D and 3D HMI images in ex vivo breast tissues could detect a benign tumor (2×5×5mm3) surrounded by normal tissue, and a malignant tumor (8×7×5mm3) embedded in glandular and fat tissues. For FUS therapy, temperature measurements and RF signals were acquired during thermal ablation. HMI images during FUS ablation showed lower displacements, indicating thus tissue hardening due to lesion formation at temperatures higher than 50°C. A finite-element model (FEM) simulation was also used to analyze the findings of the experimental results. In conclusion, this technique demonstrates feasibility of the HMI technique for tumor detection and characterization, as well as real-time monitoring of tissue ablation based on the associated tissue elasticity changes.