Magnetic order in two-dimensional systems was not supposed to exist at finite temperature.In recent years,the successful preparation of two-dimensional ferromagnetic materials such as CrI3,Cr2Ge2Te6,and Fe3GeTe2 opens up a new chapter in the remarkable field of two-dimensional materials.Here,we report on a theoretical analysis of the stability of ferromagnetism in Fe3GeTe2.We uncover the mechanism of holding long-range magnetic order and propose a model to estimate the Curie temperature of Fe3GeTe2.Our results reveal the essential role of magnetic anisotropy in maintaining the magnetic order of two-dimensional systems.The theoretical method used here can be generalized to future research of other magnetic two-dimensional systems.