We previously demonstrated the protective effects of blueberry (BB) and black raspberry (BRB) supplemented at 2.5% dose in an ACI rat mammary tumor model. Here, we assessed a dose-related alteration in tumor indices with diet supplemented with 5% BB or BRB powder. The diet was well tolerated. Tumor palpation from 12 weeks revealed first tumor appearance by 84 days in the control group, that was delayed by 24 and 39 days with the BB and BRB diets, respectively (p = 0.04). Ellagic acid detected in the plasma of rats fed the BRB diet was in the range of 96.6-294.2 ng/mL. While the BB diet showed better efficacy in reducing mammary tissue proliferation and tumor burden, tumor latency was delayed efficiently by BRB. Furthermore, BB was effective in downregulating CYP1A1 expression, while BRB downregulated ERα expression effectively. Distinct anticarcinogenic effects of the two berries correspond to their distinct phytochemical signatures.