Reliable test results that show the material characteristics of a micromaterial are necessary for the accurate analysis and design of microforming processes. The size effects in the microforming are predicted to have a significant impact on the material behavior. Two size effects are explored in metallic materials. One is the grain size effect, and the other is the feature/specimen size effect. In this study, the feature size effect on the smoothing process with the consideration of tool surface roughness is investigated numerically for metal microtubes by the flaring test. Stainless-steel (SUS 316L) microtubes with the same outer diameter of 500 μm and different wall thicknesses of 50, 25 and 10 μm were used in the FE analysis to study the feature size effect on the microscale by the flaring test. The surface roughnesses of the inner and outer surfaces of the microtube, as well as the surface asperity of the conical tool, were modeled in the cyclic concave-convex configuration. It is found, in the flaring test with using rough and fine tools, that the smoothing process on the inner surface of the microtube (ISM), as well as the plastic strain in the wall thickness of microtube, is affected owing to the rigidity of the microtube, which decreases as the wall thickness of the microtube decreases. These results suggest that the feature size affects the flaring test results for the metal microtube.