Abstract: Since non-linearity is a necessary condition for chaotic behavior, the question whether an obserVed time series consist of nonlinear ingredients is of particular interest for the fUrther analysis of the time series with the chaotic approach. This paper discusses non-linearity in the radiated noise from Underwater targets with the surrogate data method. By comparing the differences of a third-order quantity and the third-order auto-correlation between observed data and surrogate data, the non-linearity in the radiated noise is analyzed. The results conclude that there exist non-linearity ingredients in the radiated noise from underwater targets and the non-linearity is not caused by the static nonlinear transformation measurement. The results provide theoretical foundation for the analysis of the radiated noise from underwater targets with the nonlinear time series method.