The effect of brass coated steel micro-fiber on mitigating deleterious expansion due to ASR has been investigated in this research. The potentially reactive basaltic aggregate was chosen as a reactive material. Two series of specimens containing different amounts of microfiber were prepared. One of them was cured in 1 M NaOH solution at 80 degrees C, to obtain similar maturity other series were cured in 80 degrees C water up to 120 days. ASR expansion, strength development and toughness properties were observed during 120 days in NaOH solution and results compared with specimens that were kept in water. Test results indicate that steel fibers reduced ASR expansion significantly however it cannot mitigate ASR expansion completely. Furthermore, fibers are effective to prevent the mechanical properties loss due to ASR expansion such as flexural strength, compressive strength and toughness. Micro-structural investigations revealed the reaction products having different morphology (fibrous, rosette type, network appearance, etc.) when the specimens were kept in NaOH solution. (C) 2011 Elsevier Ltd. All rights reserved.