There has been extensive literature reported on the dielectric properties in the microwave frequency range (namely relative permittivity and conductivity, or loss tangent, as a function of frequency) of biological tissues in breast [6], [18]–[20]. The latest and probably the most comprehensive study by Lazebnik etal.[20]leads to a conclusion that the contrast in the dielectric properties between malignant and normal adipose-dominated breast tissues is significant. For example, at 2.3 GHz, the ratios of the relative permittivity and conductivity between malignant and normal adipose-dominated breast tissues are about 12 and 22, respectively; at 12 GHz, the ratios are 9.4 and 29, respectively.However,it has also been found that the contrast between malignant and normal glandular tissues is much less than previously thought. In this study, five groups of artificially defined breast tissues with dielectric properties ranging from low (typical fatty tissue/low water content) to high (typical malignant tissue/high water content)are studied to attain an overall under standing of the thermo-acoustic responses of breast tissues with different dielectric properties. The following one-pole ColeCole model [20] is used to define the five breast tissue groups