Thespectralpropertiesofnonlineardragforcesofrandomwavesonverticalcircularcylindersareanalyzedinthispaperbymeansofnonlinearspectralanalysis.Theanalysisprovidesbasicparametersforestimationofthecharacteristicdragforces.Numericalcomputationisalsoperformedfortheinvestigationoftheeffectsofnonlinearityofthedragforces.TheresultsindicatethatthewavedragforcescalculatedbylinearwavetheoryarelargerthanthosecalculatedbythethirdorderStokeswavetheoryforgivenwaves.Thedifferencebetweenthemincreaseswithwaveheight.Thewavedragforcescalculatedbyuseoflinearapproximationareabout5%smallerthantheiractualvalueswhenmeasuredinthepeakvaluesofspectraldensities.Thiswillresultinasafetyproblemforthedesignofoffshorestructures.Therefore,thenonlineareffectofwavedragforcesshouldbetakenintoconsiderationindesignandapplicationofimportantoffshorestructures.