In this present study fatigue properties of polypropylene gear reinforced with metallic springs was investigated. Three different springs was used as reinforcement material in the study. Involute rack gear specimens were produced with injection moulding process. Extension springs were used as reinforcement element and placed into the mould and stretched before injection of polypropylene material into the mould. After injection of polypropylene, stretched springs were loosened in order to obtain residual compression stress in the vertical section of gear tooth. Fatigue tests were performed on the produced gear. Reinforcement increased the strength of gears. Increase in the wire diameter of reinforcement spring and the amount of residual compression stress resulted in increase in service life 30 times more than that of specimens without reinforcement. At constant load cycle number, 10% increase was obtained in the strength of specimens reinforced with springs and residual compression. (c) 2004 Elsevier Ltd. All rights reserved.