In a murine model of lipopolysaccharide (LPS)-induced mastitis, Houttuynia cordata Thunb EO (HEO) and a self-microemulsion preparation of the same EO (SME-HEO), significantly downregulated pro-inflammatory factors TNF-α and IL-1β, upregulated anti-inflammatory factor IL-10, inhibited myeloperoxidase (MPO) expression, and alleviated histopathological injury in murine mammary gland tissues in a dose-dependent manner. Here, MPO is linked to mastitis.