Steels with ultrafine (α+θ) duplex structure, consisting of ferrite matrix (α) with average grainsize of about 1μm and dispersed cementite particles (θ), have been investigated widely in recent years formaking better the work-hardening capability of ultrafine-grained steels. In fact, the ratio of yield strength to tensilestrength for plain carbon steels with ultrafine (α+θ) duplex structure is commonly larger than 0.85. Forstructural material, the low ratio of yield strength to tensile strength is beneficial to absorb external energy anddelay theoccurrence of destruction. However, the ratio of yield strength to tensile strength is still relatively high for steelswith ultrafine (α+θ) duplex structure to act as the structural material. Namely, the work-hardening capability ofultrafine (α+θ) duplex steel needs further improving. It could be feasible for improving the work-hardeningcapability of ultrafine (α+θ) duplex steel to change the form, size and distribution of the cementite. Therefore, it isnecessary to investigate the work--hardening behavior of steel with different cementite states. In the present research, four different microstructures of eutectoid steel were obtained by different thermo-mechanicaltreatments, i.e., lamellar pearlite, spheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ)duplex structure. The effect of different microstructures on the room--temperature work-hardening behavior of theeutectoid steel was analyzed using room temperature tensile tests, SEM and TEM. The results indicated that thework-hardening characters of lamellar pearlite,which initial work hardening rate is large but decreases quickly with strain, have direct relationship with its large tensile strength, small yield ratio and low uniform elongation.Although the initial work hardening rates of the three ferrite/cementite particles duplex structures are lower, theydecrease much slowly with strain comparing with that of lamellar pearlite. Therefore, three types offerrite/cementite particles duplex structures demonstrate good plastic deformation capability. In comparison withspheroidized pearlite, ultrafine (α+θ) duplex structure and fine-grained (α+θ) duplex structure demonstrate better balance between strength and plasticity due to microstructure refinement.