CMR T1 and ECV quantification can be used to characterize focal or diffuse myocardial fibrosis. However, it is technically challenging to acquire high-quality maps in small animals for preclinical research because of high heart rates and high respiration rates. In this study, we developed an ECG-free, free-breathing MR Multitasking T1 mapping method on a 9.4T small animal MRI system. The feasibility of characterizing diffuse myocardial fibrosis was tested in a HFpEF rat model. Elevated ECV found in the HFpEF group is consistent with previous human studies and shows strong correlation with the histological data.