This study presents numerical simulations of a cross-flow vertical-axis marine current turbine (straight-bladed Darrieus type) with particular emphasis on rotor-performance prediction and hydrodynamic characteristics. Numerical investigations of a model turbine (power coefficient and flow behaviour) were undertaken using developed computational models. Turbine design was studied using a time-accurate Reynolds-averaged Navier-Stokes (RANS) commercial solver (Fluent v. 6.3). A physical transient-rotor-stator model with a sliding mesh technique was used to capture change in flow field at a particular time step. A shear stress-transport k-u turbulence model was used to model turbulent features of the flow. Developed model can effectively predict hydrodynamic performance of a vertical-axis marine current turbine.