Abstract: The hardness versus time curves during aging at 450-650℃ of Cu-Nb steels with various copper content have been measured. The precipitation of ε-Cu, Nb carbonitfide and Cr carbide found during aging contributes to the hardening. The result indicates that the Cr carbide hardness peak emerges after long time aging. The more is the copper content the earlier the hardness peak of ε-Cu occurs. In high copper steel the hardness peaks of ε-Cu and Nb carbonitride separates and in low copper steel they overlaps.