Sensing acoustically induced local harmonic motion using a microwave transceiver system may provide useful information for detecting nonpalpable tumors in dense breast tissue. For this purpose, we propose the harmonic motion microwave Doppler imaging method, in which the first harmonic of the phase modulated signal due to local harmonic motion is sensed. This signal is related to the dielectric, elastic, and acoustic properties of the vibrating region. The purpose of this paper is twofold: 1) to demonstrate the concept of this method with experiments using phantom materials mimicking the elastic and electrical properties of the breast tissue and 2) to investigate the effect of fibroglandular region size and vibration frequency on the received signal, using numerical simulations. A breast phantom with a tumor phantom inclusion (5-mm diameter and 7-mm height) inside fibroglandular region is constructed for the experimental study. The response due to a focused ultrasound probe is linearly scanned at 30-mm depth from the phantom surface, and the Doppler signal level is tracked using a spectrum analyzer. It is shown that the tumor phantom is resolvable inside the surrounding fibroglandular region with about a 3-5-dB decrease in the signal level. The simulations, using the finite-difference time-domain method, show that the received signal level depends on the relative size of the fibroglandular region with respect to the vibrating region size. Further experimental and numerical studies are needed to investigate the feasibility of this method and to optimize the imaging system design.