Recent tension leg platform (TLP) model tests have revealed strong transient resonant responses during extreme wave frequency responses. It is said that these responses, called ringing, are associated with higher order forces induced by large-crested, near breaking waves. In this paper, as a second step of giving light on the occurrence mechanism of such nonlinear phenomenon, third order wave force measurement test are carried out in regular and irregular waves. A third order spectral analysis method (the so-called Cross tri-spectral analysis) is newly developed to estimate the second and third order transfer functions of wave loads on TLP. And the numerical calculation method based on the third order diffraction theory (Malenica and Molin theory) has been developed for a vertical uniform cylinder. Both the calculation result and the approximate solution for a vertical uniform cylinder, which is derived by Faltinsen-Newman and Vinje (low ka -low KA theory : k ; wave number, a, cylinder radius, A ; wave height), are used to evaluate approximately the third order force on TLP under the assumption taking only the phase difference based on the column distance of TLP into account. As a result, the present higher order spectral analysis method is much effective for identifying the higher order transfer functions from wave force data in irregular waves. However, the approximate methods used for estimating the third order force on TLP underpredict against the experimental results. It is necessary to take account of some interaction effect between columns.