This paper deals with the surface finishing of cooling channels in die casting dies with free abrasive grains. A baffle plate was set in the cooling channel in order to make a path of the free abrasive grains. The influence of plate length on finishing performance was investigated experimentally. The distance between the plate edge and the spot channel tip was also evaluated in order to optimize the finishing performance. As results, it was possible to improve the surface roughness at spot cooling channel by applying the baffle plate. However, hardening surface remained on the internal face in the cooling channel along the baffle plate. When the distance between the plate edge and the spot channel tip was 1 mm, the chisel edge at the spot cooling channel was rounded by the free abrasive grains.