Genetic manipulations have enabled the mouse to be widely used as an animal model for investigating the mechanisms of human atherosclerotic disease. However, there is no standard method for quantifying atherosclerotic lesions among different laboratories. The present study introduces a thorough and precise quantitative assessment of atherosclerotic lesions in mice. In the present study, 6‑week‑old apoE‑/‑mice were fed either a chow diet or a high‑fat diet (HFD) for 12 weeks. Plasma lipid levels were measured every four weeks. Aortic atherosclerotic lesions were quantified and analyzed using an image analysis system. The aortic tree was isolated and stained with Oil Red O to measure the gross lesion area. The heart was removed and divided into sequential cross sections, which were then assessed for microscopic intimal lesions in the aortic root as follows:(1) Elastic van Gieson staining was performed to determine the area of the atherosclerotic lesion;(2) cross sections were stained with hematoxylin and eosin for histological analysis; and (3) cross sections were stained with Oil Red O and immunohistochemical staining for quantitative analysis of the cellular components within the lesions. ApoE‑/‑mice fed with either the chow diet or HFD developed severe atherosclerosis in the aortic root, however, there were few lesions in the remainder of the aortic tree. Compared with the control group, the HFD apoE‑/‑mice had increased plasma lipid levels and increases in the gross lesion area in the aortic tree, the microscopic lesion area in the aortic root and the number of macrophages, vascular smooth muscle cells and neutral lipids present within the lesions. HFD feeding in the apoE‑/‑mice accelerated the development of atherosclerosis. The quantitative method described in the present study may be used to assist in future investigations of atherosclerosis in mice.