Apelin is a 36 amino-acid vasoactive peptide originally isolated from bovine stomach extracts as a selective endogenous ligand of orphan receptor, APJ, which was genetically identified to have closest identity to the angiotensin II type 1 receptor (AT-1). Previous studies have shown that both apelin and its receptor, APJ, distribute in a wide variety of tissues including central nervus system, heart, lungs, and kidneys, and apelin-APJ system has been found to have relevant roles in human physiology, such as the regulation of cardiovascular, gastrointestinal, and immune functions, bone physiology, fluid homeostasis, and embryonal development of cardiovascular system. Interestingly, in spite of high homology between APJ receptor and AT-1 receptor, as well as high similarity in the tissue expression of both receptors, previous reports showed opposing actions between apelin-APJ system and angiotensin II (Ang II)-AT-1 system in a number of physiologic and patho-physiologic settings. In this mini-review, we highlight the actions of these two systems, and discuss the physiologic and pathophysiologic significance of these systems.