  Global quantum discord (GQD), proposed by Rulli and Sarandy [Phys. Rev. A
\textbf{84}, 042109 (2011)], is a generalization of quantum discord to
multipartite states. In this paper, we provide an equivalent expression for
GQD, and obtain the analytical expressions of GQD for two classes of
multi-qubit states. The phenomena of sudden transition and freeze of GQD are
also discussed.
