The effects of C and N on the work-hardening behaviors were compared in metastable austenitic steels in which varied amounts of C and N were separately added (Fe-18%Cr-8%Ni-(C,N) alloys). Although both C and N suppressed deformation-induced martensitic transformation during tensile deformation due to their austenite-stabilizing effect, they enhanced the work hardening of the steels. Comparison of C-added and N-added steels revealed that C addition more increased the work-hardening rate than N addition. In order to clarify the reason of the more significant effect of C, the individual hardness of deformed austenite and deformation-induced martensite (DIM) were measured in cold-rolled C-added and N-added steels by using a nano-indentation tester. The nanohardness of deformed austenite increased with increasing the thickness reduction and amount of added C and N. However, there is little difference between C-added and N-added steels in the hardening behavior of austenite, meaning that the difference in work-hardening rate of metastable austenitic steel between C-added and N-added steels is not derived from the hardness of deformed austenite but that of DIM. The nanohardness of DIM was significantly higher in the C-added steel than N-added steel, and thus, the main factor affecting the higher work hardening of 0.1C steel should be considered to be the higher hardness of C-containing DIM. In addition, in C-added steels, an excellent strength-ductility balance was achieved compared with N-added steel because the hard DIM is gradually formed until the later stage of deformation, meaning that pronounced TRIP effect was obtained in C-added steels than N-added steels.