The microstructure change by warm deformation in low alloy steels with different initial ferrite (α) + cementite (θ) duplex structures is discussed in the present paper. In high carbon steels, heterogeneous deformation introduced in pearlite containing lamellar θ promotes dynamic recrystallization (DRX) of α for mild deformation of less than 1.2 in true strain. On the other hand, the original α grains become elongated and only subgrains are formed by dynamic recovery in the case of (α + θ) duplex structure containing equiaxed spheroidized θ. Equiaxed fine α grains, approximately 2 μm in diameter and mostly bounded by high-angle boundaries, are formed with spheroidized θ by DRX during compression of the pearlite by 75%. When the (α + θ) duplex structure containing spheroidized θ was deformed, the original α grains become elongated and only subgrains are formed by dynamic recovery. For the tempered martensite, equiaxed α grains similar to those in the deformed pearlite were obtained after 50% compression. This indicates that the critical strain needed for the completion of DRX is smaller for the tempered martensite than for the other structures. Lath martensite in a higher carbon alloy is more suitable for DRX because of its finer initial grain size. DRX α grain size is finer in a higher carbon alloy because of stronger pinning effect by θ particle.