Two mouse models of mammary cancer (a carcinoma and sarcoma) were examined using quantitative ultrasound (QUS) and three models for ultrasound backscattering. The first model that was examined was the spherical Gaussian model (SGM). The second model was the fluid-filled sphere model (FFSM) and was hypothesized to model scattering from cell nuclei with the cell cytoplasm acting as background. The third model, called the new cell model (NCM), was constructed to model backscattering from cell nuclei and cytoskeleton. The average scatterer diameters (ASDs) were estimated from regions-of-interest (ROIs) inside the tumors using all three models and then compared. The ultrasound analysis bandwidth used in the study was 16 to 27 MHz. QUS images of the tumors utilizing the ASD estimates from the three models were constructed. The ASDs were 30.3 ± 3.06 μm and 25.2 ± 4.01 μm with the SGM, 47.3 ± 7.99 μm and 47.7 ± 7.01 μm with the FFSM and 41.2 ± 1.39 μm and 34.4 ± 5.95 μm with the NCM for the carcinoma and sarcoma, respectively. statistically significant differences between the ASD estimates from the carcinomas and sarcomas were observed using the SGM and NCM but not with the FFSM.