The helicopter power transmission system technology is the key technical area for improving the helicopter performance, reducing the noise/vibration level of helicopters and decreasing the cost of life cycle of helicopters. In this paper, the technical characteristics of the helicopter power transmission system are introduced first. Then, the development history and trend of the transmission configuration, the component and the design and analysis technique of the transmission system are described. The advanced material and process technology applied in the helicopter power transmission system are also described. Finally, the power transmission system technology used in the high speed helicopters is briefly presented.