In order to distinguish the difference between AISI 321 stainless steel and Hadfield steel in work hardening behavior, both the Hollomon analysis and the differential Crussard-Jaoul analysis were used to determine the strain hardening exponent as a function of the strain. The results showed that the differential Crussard-Jaoul analysis characterized the discrepancy between AISI 321 steel and Hadfield steel in work hardening behavior more accurately than the Hollomon analysis. The work hardening of AISI 321 stainless steel resulted mainly from interactions of dislocations. When the true strain was rather low, the work hardening of Hadfield steel also resulted mainly from interactions of dislocations. At high strains, twinning would occur in Hadfield steel. It was the occurrence of twins that led to unusual work hardening at larger strains in Hadfield steel.