A micro-duplex structure consisting of austenite and ferrite was produced by equal channel angular pressing and subsequent intercritical annealing. As compared to coarse-grained (CG) counterpart, the strength and ductility of micro-duplex samples are enhanced simultaneously due to smaller grain sizes in both phases and more uniformly distributed austenite in ferrite matrix. The average yield stress and uniform elongation are increased to 540 MPa and 0.3 as compared to 403MPa and 0.26 of its CG counterpart respectively. The Hall-Petch coefficients of austenite and ferrite grain boundaries were quantitatively measured as 224.9 and 428.9 MPa mu m(1/2) respectively. In addition, a Hall-Petch type coefficient was used to describe the ability of phase boundary to obstruct dislocation motion, which was measured as 309.7 MPa mu m(1/2). Furthermore, the surface-to-volume ratio of phase boundary in micro-duplex structure was estimated to be 1.17 x 10(6) m(-1), which is increased by an order of magnitude as compared to 1.2 x 10(5) m(-1) of its CG counterpart. Based on the strain gradient theory, a model was proposed to describe the effect of surface-to-volume ratio of phase boundary on strain hardening rate, which shows a good agreement with the experimental results.