Effect of semicircular rib or groove on the performance of a rim-seal was evaluated using three-dimensional Reynolds-averaged Navier-Stokes equations. The turbulence was modeled using the one-equation Spalart-Allmaras turbulence closure model. Reynolds number based on the axial chord of the turbine blade was 500,000 at the mainstream outlet. The numerical results for a sealing effectiveness was validated in comparison with experimental data. To examine the effect of the semicircular rib or groove on sealing effectiveness of the rim-seal, location and diameter of the rib or groove were selected as the parameters to be performed. The rim-seals with the semicircular groove showed a higher sealing effectiveness than that with the semicircular rib. The semicircular groove installed on the stator side showed best sealing effectiveness among the numerical simulation results.