In this study, improvement of impact toughness of gray cast iron by reinforcing steel fiber was investigated. The composite material was produced by sand mould casting technique. Then, normalizing heat treatment was applied to the specimens at various temperatures (800 °C, 850 °C, and 900 °C). Charpy impact toughness tests were conducted at the temperature of -80 °C, 0 °C, and 27 °C. The results show that impact toughness of the gray cast iron is increased by reinforcing low carbon steel wire. As the test temperature is decreased, impact toughness of the composite specimens decreases considerably. It is noted that graphite free transition regions with high hardness were observed due to the carbon diffusion from cast iron to steel fiber. It is considered that interphase (fine pearlitic phase without graphite flakes) and bond quality in transition region contribute to increase in impact toughness of the cast iron composites. © 2009 Elsevier B.V. All rights reserved.