In order to clarify the mechanism of flowability improvement by mold surface roughness on Poly Carbonate thin-wall injection molding, we analyzed the replication ratio and the pressure in a mold on short-shot molded pieces. In addition, we observed particularly the internal morphology. The mold surface used was roughened by using electro-discharge machining to obtain an isotropic surface profile, and the molds that had four kinds of surface roughness were provided for this study. The replication ratio was increased by increasing the resin pressure in the mold when the mold surface roughness was fine, and the filling into the concave-convex shape was completed smoothly. On the other hand, the replication ratio was increased when the resin pressure increased at a slow rate in the mold when the mold surface roughness was rough, and the filling on the concave-convex shape was completed at a stretch after maintaining the replication ratio at approximately 90%. Also, in polarized observation, when the mold surface roughness (Ra) was 0.18μm or less, the higher-order structure of the molded piece was a skin layer, a shear-oriented-layer, and a core layer from the surface. On the other hand, when the mold surface roughness (Ra) was 0.47μm or more, the coexistence of a skin layer and a shear-oriented-layer was confirmed depending on the surface shapes at the surface. From these results, we clarified the effect on the molten polymer flow by the surface roughness.