<p>This study proposes using multifunction cavitation (MFC) to strengthen the surface of helical gears used in transmissions. In addition, because the steel used for helical gears exhibits poor corrosion resistance in water, samples plated with electroless nickel and iron tetroxide were also investigated. In addition to using samples with modified surfaces, we used samples strengthened by carburizing and nitriding. The noncoated and plated samples showed a slight increase in surface roughness and increased compressive residual stress and hardness. The plating method that did not cause red rust during the MFC treatment was iron tetroxide plating. In the samples strengthened by carburizing and nitriding, red rust did not occur on the surface after MFC treatment, the surface roughness was suppressed, and compressive residual stress and hardness increased. The results show that the MFC treatment effectively improved the fatigue properties of gear surfaces with complex shapes.</p>