Hydrodynamic derivatives including the effect of frequency are estimated by means of. transient manoeuvring tests. Planar motion mechanism (PMM) was used for this purpose, but partially changed to allow transient motions. Utilizing the Fourier analysis, linear derivatives are obtained in wide frequency range and they coincide with those derived from regular PMM tests. But the reliability of the results deeply depends on the characteristics of input transient motions and on the quality of measured forces. Then, the effect of maximum amplitude and time parameters applied to the transient motions are studied. Each pairs of derivatives (e. g. Y _υ and Y _υ, N _r and N _r.) satisfied the Kramers-Kronig relationship in nature. First, Hilbert transform was applied to the experiment results, and the relationship was confirmed in general. But, there are unnegligible differences, because of the limitation in measured frequency range. Next, supposing the causal system, each transfer function is assumed to a rational function with a finite order of Hurwitz polynomials. If a proper set of coefficients in the transfer function is identified, we need no more to confirm the Kramers-Kronig relationship. Bode's diagram of the transfer function is used for parameter identification, and each experiment result is represented by Eq. (4. 6) fairly well