Abstract Polymer modified steel fiber-reinforced concretes were produced with addition of both steel fibers and a styrene butadiene rubber emulsion (SBR). Both flexural and compressive strength of the composites after 28 days curing were tested. Microstructures of the composites were analyzed by using scanning electron microscope and mercury intrusion porosimetry. Results show that the addition of steel fibers increases both flexural and compressive strength of the composites. The flexural strength increases significantly when containing 3–10 wt.% SBR. The optimal use of SBR is 5 wt.%. However, the compressive strength may decrease with the addition of SBR. When the addition arrives 10 wt.%, a 16% reduction is observed. The overall porosity and pore size distribution of the composites vary with SBR content. The addition of 3 or 5 wt.% SBR can refine the pore size distribution. Interweaving polymer films were observed in the composites.