  The objective of this paper is to study the local time and Tanaka formula of
symmetric $G$-martingales. We introduce the local time of $G$-martingales and
show that they belong to $G$-expectation space $L_{G}^{2}(\Omega _{T})$. The
bicontinuous modification of local time is obtained. We finally give the Tanaka
formula for convex functions of $G$-martingales.
