Atherosclerosis is an inflammatory disease caused by lipid. Monocytes and their descendant macrophages play an important role in the occurrence and development of atherosclerosis. Macrophages become foam cells after lipid-laden, and foam cells form the lipid stripes and plaques of atherosclerosis. Over the past several years, the understanding of how monocytes accumulate in the growing lesion, differentiate, ingest lipids, and their roles in atherosclerosis has advanced substantially. Monocyte/macrophage's phenotype and functional complexity suggests that it is likely to become a new target for the treatment of atherosclerosis. Monocyte/macrophage's behavior changes in the process of atherosclerosis are reviewed in this article.