Abstract: A gear transmission system with a helicopter rotor was experimentally investigated and on-line identified during its operation,and the results were compared with its theoretical analysis.It is found that the gear transmission system with a rotor differs from other gear transmission system by such natural characteristics,that its dynamic characteristics change with it's rotational speed.It's changing law has also been obtained,which provides basis for the dynamic modeling analysis and the dynamic design of the system and an effective analytical approach of the gear transmission system.