High density plasma nitriding system and procedure had been developed for solid-solution hardening of stainless steels and tool steels at low temperature. In particular, high content of nitrogen atoms up to 40 at% were infiltrated in the depth of AISI420 martensitic stainless steels down to 30 µm. This resulted in significant increase of hardness because of solid-solution strengthening process without any precipitation reactions to nitrides. In the present paper, initial micro-dot and micro-line patterns were first printed onto the surface of polished stainless steel mold by using the dispenser. This masked mold substrate was nitrided at 693 K for 7.2 ks at 70 Pa by using the high density plasma nitriding system. The masked part was never nitrided to maintain the same hardness as the matrix; the unmasked parts were selectively nitrided to have higher hardness than 1000 Hv. That is, the initial masking pattern was transformed to hardness profile pattern on the mold surface. This hardness-patterned mold-die was mechanically sand-blasted. Since the masked parts of mold-die surface were selectively removed by this blasting, the original mask pattern was machined into the mold-die. This microtextured mold-die geometry was analyzed by using the surface profilometer and SEM to demonstrate that this processed mold-die was utilized as a mold unit for injection molding. Micro-bump patterned plastic cover-case for cellular phones were injection-molded by using this method for practical demonstration.