Numerical simulations are conducted to investigate the effect of an augmentation channel on the performance of a straight-blade vertical-axis turbine. The flow field is simulated with a commercial software FLUENT, in which the finite volume method is used to solve Reynolds averaged Navier-Stokes equations and the SST k-ω turbulence model. The rotation of the rotor is simulated by solving a rotation motion equation. A comparison between a standalone turbine in a free, open stream versus one in an augmentation channel is drawn, with discussion regarding variation of hydrodynamic torques on the blade and rotor, the power and torque output by the shaft, and the rotational speed. The comparison reveals that for a turbine in a channel, the fluctuations of hydrodynamic torques and rotational speed are dramatically reduced, and the power output is more than 30% higher than that of the standalone turbine.