Abstract: A gear structure of servo has a control body, a casing and a reduction gear set. The control body has a driving motor. The casing is mounted and covered on the control body and has a receiving space formed between the casing and the control body. The reduction gear set is mounted in the receiving space and has a first gear unit, a second gear unit, a third gear unit and a fourth gear unit. Each one of the gear units engages a corresponding gear unit. Helical gears are used in the reduction gear set, so the abrasion and noises caused by the engagements between the gear units can be reduced via the helical gears, and promote the transmitting efficiency of the driving motor.