  When combined with transition metals with partially filled $d$-orbitals,
magnetism can be incorporated in two-dimensional materials, which greatly
expands the scope for fundamental researches and potential applications of
these materials. Here, a first-principles study of a new two-dimensional
ferromagnet VI$_3$ has been carried out. The structural symmetry, magnetic and
electronic properties of VI$_3$ in its bulk and single layer forms have been
confirmed and predicted, respectively. Its ferromagnetic Curie temperature is
predicted to be reduced by half in its monolayer form. In addition, the cation
substitution in its monolayer have also been studied, which can significantly
tune the magnetism.
