  We explore the electronic structure, orbital character and topological aspect
of a monolayer MoS$_2$ nanoribbon using tight-binding (TB) and low-energy
(${\bm k}\cdot{\bm p} $) models. We obtain a mid-gap edge mode in the zigzag
ribbon of monolayer MoS$_2$, which can be traced back to the topological
properties of the bulk band structure. Monolayer MoS$_2$ can be considered as a
valley Hall insulator. The boundary conditions at armchair edges mix the
valleys on the edges, and a gap is induced in the edge modes. The spin-orbit
coupling in the valence band reduces the hybridization of the bulk states.
