Abstract: In order to decrease the numbers of identificationparameters and overcome difficulties in computation, the rank of structureshould be reduced at first. In this paper, the rank of structuremodel is reduced by K-L(Karhunen-Loeve) eigenvalue extracting method.Modal parameters of the reduced model are identified by the least squaremethod. The number of identification parameters and the computing complexity aregreatly reduced.A simple girder is analyzed as an example. A three rank model isobtained. The identified inherent frequencies and the ratios of modal dampingare close to the analytic result.