The present study reports a case of a 5-month-old female adrenomedullin (AM) heterozygous (+/-) mouse that presented a mass of leiomyosarcoma found in the right shoulder girdle region. The neoplastic mass extended to the sternal region and showed hemorrhages, congestion and necrotic foci. The excised tumor with a diameter of 2.5cm was firm, ill-demarcated and had focally infiltrated the surrounding muscles. The cut surface was homogeneously whitish with multi-focal reddish lesions. Microscopically, the tumor composed of variable fascicles of spindle-shaped cells with pleomorphic and cigar-shaped nuclei. The nuclei were round and elongated. Metastasis of tumor cell to skeletal muscle was frequently observed. Immunohistochemically, desmin, vimentin and alpha-smooth muscle actin (alpha-SMA) were demonstrated in neoplastic cells but tumor cells were negative for cytokeratin (CK) and S-100. Based on gross finding, microscopical examination and immunohistochemistry, the present case was diagnosed as a subcutaneous leiomyosarcoma.