Abstract: The theory of multiple input-output frequency response functions estimation is studied based on the finite Fourier transform. Several possible input signal types are discussed. Of them, the deterministic signal excitation is a new technique which is adrantageous over the currently prevailing random exci tation in test time, the amount of data required to be sampled and reduced and the estimation accuracy. Moreover, the hardware requirement is simpler and it is possible to run with a dual-channel FFT analyser only.In light of the theory, a software package for 6 input-multiple output FRF estimation has been developed, running in the PDP11/23 mini computer system with 64K bytes memory. The operator is prompted to select one from 4 excitation types, （ 1 ）. deterministic, （ 2 ）. periodic random, （ 3 ）. burst random, and（ 4 ）. continuous random excitation. The package was used for real structures such as a free-free beam and a helecopter tail rotor. The results show that the deterministic excitation technique was the best.