Abstract: Load identifications are to calculate applied loads by using motion equations and measured data of motion parameters of a system. They are inverse problems of solving differential equations. The first part of this paper presents the basic theory of the frequency response function method in load identifications. The calculation formulas areH（w）F（w）=X（w） andSx（w） = H（w）Sf（w）H*T（w）which are suitable for different loads. Random loads are discussed for the 1st time in load identifications, so that the application field of load identifications is enlarged. The second part of this paper presents in details the six-force-factor identification of helicopters. Three main aspects about six-force-factor identifications are described respectively. They are the calibration on the ground （the measurement of transfer matrix）, the measurement of the structrual response in flight and the calculation of six-force-factor （solving Eq. ）. Strains are for the first time chosen as the measurement parameters in the load identification. Not only the dynamic load but also the static load and the fatigue test data can be obtained at one time. Practice has proved that the direct measurement method of the strain stransfer matrix is simple and reliable if the number and locations of the measurement points are chosen rationally. As to measurement of moment transfer functions using eccentric forces, this paper gives expressions of transfomation matrices. The influence of the data processing and the method of solving equations on the results is also discussed in this paper. The six-force-factor of helicopters has been identified successfully. The data obtained can be used in design of the helicopters,