Epidemiological studies show increased risk of breast cancer associated with Western diets containing high animal fat. The aim of this study was to evaluate changes in mammary fat composition due to a high animal fat diet (HAFD), compared to a low fat diet (LFD), in a mouse model of breast cancer. Localized MR spectroscopy and high spectral and spatial resolution (HiSS) MRI data were acquired at 9.4T. The saturated fat fractions increased and the polyunsaturated bond component decreased significantly in HAFD mice compared to LFD mice. Fat images obtained from HiSS demonstrated increased mammary fat concentration in HAFD mice than LED mice.