Microwave-induced thermoacoustic imaging (TAI), combining the high contrast of microwave imaging and high resolution of ultrasound imaging, is a promising noninvasive imaging technique for breast cancer detection (R. A. Kruger et al, Radiology, vol. 216, 279-283, 2000). In order to efficiently detect a breast tumor by TAI, understanding the characteristics of the acoustic signal emanated by a tumor target is needed. Theories of thermoacoustic signal generation have been well established. The characteristics of the induced acoustic signal are principally contributed by three sets of factors: first, features of the microwave pulse including width, waveform (envelope of the microwave pulse) and carrier frequency; second, tumor dimension and morphology; and third, dielectric, acoustic and thermal properties of the biological sample containing both the tumor and background normal tissues. Here, our interest is focused on studying different pulse widths, pulse waveforms and tumor sizes. Although some effects of pulse width and target size have been reported, impact of pulse waveform and especially the combination of pulse width, pulse waveform and target size has not been addressed by previous research endeavors.