Magnesium is a major terrestrial rock element and plays an important role in the global geochemical cycles. In terrestrial waters, dissolved magnesium is derived from the weathering of both siliceous and carbonate rocks. It is then transported to the ocean where it is crucial for the aquatic biota and regulates global climate over geologic time scales. Recently, a technologically advanced multicollector inductively coupled plasma mass spectrometry provided an opportunity to measure the magnesium isotope ratio (δ26Mg) with high accuracy. In this paper we review ongoing researches on the δ26Mg value in the river systems over the world, as well as its controlling factors, including isotope fractionation during chemical weathering, secondary mineral formation, and biological activity. Furthermore, we examine the potential use of δ26Mg as a new tool for the better understanding of chemical weathering processes and the global magnesium cycle, which ultimately controls the Earth's surface environment.