Theroleofnuclearweapon in theKorean peninsula had been changed bytheU.Snuclearpolicyfor33yearsfrom 1958to1991.Previousstudies on U.S nuclearpolicy on the Korean peninsula were analysis with U.S hegemony and biased critiqueon theU.S.policy.So they had been not enough the objectivity of purpose of nuclear policy on the Korean peninsula.Thispaperisastudyabouttheroleoftacticalnuclearweapons on the Korean peninsula,focusing on the U.S.cognition ofthreats in North-EastAsiaandsecurityofSouthKorea. We could find the change in the role oftacticalnuclearweapons by analyzingthechangeoftheU.S.nuclearpolicy on theKorean peninsula along with U.S. cognition of threats in North-East Asia. U.S. Tactical nuclearweaponshad served forthemilitary purposewhich supplemented the conventionalforces until1960s,and then therole oftacticalnuclear weaponshad changed totheservepoliticalrolebecauseofthechangeof securitysituationinNorth-EastAsiaafter1970s. In the latterhalfof1950s,U.S.adopted the strategy which focus on nuclearforcesto decreasetheburden ofmilitary budgetin South Korea,and improvethemilitaryforcesand securethesafetyin South-EastAsia. So,relativelytheimportanceofconventionalforceswashalved. This trend had resulted in some changes. While Korea army had decreased two divisions in 1958,U.S.army in Korea was reinforced by pentomic divisions and had deployed tacticalunclearweapons as added militaryforcestoconfronttheNorthKorea-Chinacoalitionforces. Atthattime, tacticalunclear weapons in the Korean peninsula had meaningtowhattohaveeconomicalefficiencybyreplacingtacticalnuclear weaponswithconventionalforces,ratherthanitwascausedbythechange ofthreatsin Korean peninsula.Thisconcepthad been theevidencethat tacticalunclearweaponshad had themilitary roleand could beused in war. In 1970s,U.S.nuclear policy on the Korean peninsula were different from theconceptthattacticalnuclearweaponsreplaceconventionalforces becauseNixon Doctrineand SchlesingerDoctrineallowed nuclearweapon's militaryroletodecrease. Improvementin the relationship between U.S. and China caused the change in Chinese threat.U.S.had promoted nuclearpolicy to decrease tacticalnuclear weapons in South Korea and exclude it's military role,eliminating the threat of China and recognizing the only threat of conventionalforces ofNorth Korea to the Korean peninsula.This led to change oftacticalnuclearweapons'doctrine so thatpossibility ofusing tacticalnuclearweapondecreased. In the middle of1970s,U.S.governmenthad searched forpulling the troopsoutoftheKorean peninsula becausethey had believed thatthere was the equilibrium in military forces between North Korea and South Korea.And then they had recognized thatthey had underestimated the threatofNorth Korea and they had canceled thewithdrawalplan.As a result,U.S.army in South Korea had relocated orstored tacticalnuclear weapons in the rear and decreased the number of war-heads and ground-to-groundmissiles. In a word,theroleofU.S.tacticalnuclearweaponshadbeen changed from militarytopolitical.In thelatterhalfof1950s,U.Shadusedtactical nuclear weapons for military purpose to supplementconventionalforces againstthe coalition forces ofChina and North Korea.After1970s,U.S. had changed itsnuclearpolicyemphasizing“Nofirstuse"forthepolitical role,becausetheyhadexcludedthreatofChinainSouth-EastAsia. Inotherwords,U.Shadbelievedthatonlyoneroundoftacticalnuclea weaponscouldtriggerthegeneralnuclearwarandthisbeliefwasreflected on their nuclear policy.They had controlthreatofaround the Korean peninsulabyimprovingrelationshipbetween U.S.andChinaandeasingof tensionswithSovietUnion.Theyhadusedtacticalnuclearweaponsnotas powerfulweaponsinwarbutasthemethodtocontrolthethreat.