Mercury (Hg) is a globally distributed and highly toxic pollutant in the environment. Its mobility and biomagnification in aquatic food chains depend on its biogeochemistry and redox cycling. Hg isotope analysis is an important new tool for identifying Hg source and tracking Hg transformations in the environment. This review summarizes the following 4 points to endorse the Hg isotope research; 1) definition of Hg delta (δ) values and Hg standards, 2) methods of Hg isotopic measurement, 3) Hg isotopic fractionation, and 4) natural Hg isotope variation.