Yield stress ry is evaluated in a duplex steel with a + c lamellar structure by calculating the stress for the propagation of pile-up dislocations in one phase into another phase. Effective grain size of a lamella is determined to calculate the contribution of each boundary of a lamella to dislocation pile-up. Comparison of stresses required to drive a-to-c and c-to-a propagations of pile-up dislocations suggests that a + c lamellar structure yields by propagation of pile-up dislocations in a lamella to c lamella. ry of a + c lamellar structure is calculated as the sum of friction stress roa of a and the stress component Drc for yielding of c lamella at the effective grain size da,eff of a lamella. The calculated results explain better to the experimental ry than do results calculated using rule of mixture.