The UN and NATO have cooperated with each other within the sphere of peace operations. The most important elements of the UN PKO are the soldiers, police officers and civilian experts. Their operations have changed from inactive monitoring and reporting to law enforcement, called the multi-dimensional PKO. In these days,the UN considers the Security Sector Reform (SSR) as an exit strategy. The security sector includes the military forces, police, corrections, private security etc. The police function not only its own territory, but also act as a link in the network of the security sector. Therefore, the role of police is growing in the UN PKO. 40)NATO has also been acting as the peace keeper of the world since the end of the cold war. The Principle member states of NATO prefer strong military operations to inactive peace operations, such as monitoring, reporting and observing. However, the peace operations of NATO have experienced difficulty in maintaining public order, when using soldiers only. To solve the difficulty of maintaining public order, the State Department of America has organized the Civil Response Corps (CRC) with the cooperation of eight other organizations, such as the Department of Justice, the Department of Health and Human Services, and the US Agency of International Development (USAID), etc., since 2008. William Durch said that the CRC is functioning as the police in the high risky conflict areas. Therefore,researchers notice that the role of police has also become important in the peace operations by NATO. On the other hand, OECD has also recognized the necessity of the security sector practitioners of the ODA have found that the International Development is not realistic without the Security Sector Reform. Therefore, SSR is being the connection between the peace operations and the international development. The difference is that the Security Sector Reform in the peace operations is more practical and on the spot. However, Korean police have not prepared well enough. There are few researches which have been accumulated. In addition, Korean police have participated in a few missions such as East-Timor and Afghanistan. However, these experiences have been valuable. The East-Timor case is the representative mission of the third generation of the PKO which the peace operators can enforce the law and maintain public order directly within the fragile states. Police work is always on the spot and those training is also on the job. Either of classroom instruction or planning cannot get enough of the police reforming. Therefore, third generation of the PKO, like East-Timor mission, can be effective in the police reform. The Afghanistan case is the non-UN PKO led by NATO. As a mission of NATO, the Korean army, Korean police and Korea International Cooperation Agency (KOICA) have cooperated with each other. The researchers notice that this cooperating model fits for peace operations and international development. Police should play a more important role in this model.