The Middle East is one of the most unstable regions in the world and the contradiction between Palestine and Israel is the core issue. In this article we will mainly focus on the Israeli-Palestinian conflict as the biggest threat of international security in 21st century: the first section presents, including the origin of Israeli-Palestinian conflict, the legacy of Arab-Israeli wars and Ideological difference that led to conflicts in Middle East and the second section elaborates the causes of Israeli-Palestinian conflict in 21st century: the roles of nuclear weapons, water resources and US intervention in intensifying the contradictions. After a review of Peace Process, the article will end up with an explanation of the factors that contribute to the suspension of Peace Process.