Upon the explosion of technical advances over years, the number and sophistication of cyber-attacks against states showed a significant increase. A well-known example is cyber attacks against Estonia in 2007 or the Stuxnet attack against Iran in 2010. It is hardly surprising that cyberattacks have become a concern of the international community because of the legal issues surrounding their nature and their negative impact on the security of states. In the current context, a question raises is whether a country that suffers from cyber-attacks can activate the right to self-defense under Article 51 of the United Nations charter whereas both customary international law and international treaties have not been updated about the answers yet. Some famous scholars have already provided their ideas about this issue and come to the conclusion of accepting the right to self-defense of victim states. However, those approaches should be examined and reexamined continuously. This thesis will attempt to unlock the above question and contribute more legal analysis relevant to the right of self-defense in case one state is attacked by cyber-attacks. The thesis is divided into three parts. The first part deals with questions like: What is self-defense? When and under what conditions, self-defense may legitimately take place. In the following, the focus of the discussion in the second part is defining the acceptable concept and the characteristics of cyber-attacks. Lastly, the third part wrestles with the most controversial topic that whether cyber-attacks can activate the right to self-defense under Article 51 UN Charter.Upon the explosion of technical advances over years, the number and sophistication of cyber-attacks against states showed a significant increase. A well-known example is cyber attacks against Estonia in 2007 or the Stuxnet attack against Iran in 2010. It is hardly surprising that cyberattacks have become a concern of the international community because of the legal issues surrounding their nature and their negative impact on the security of states. In the current context, a question raises is whether a country that suffers from cyber-attacks can activate the right to self-defense under Article 51 of the United Nations charter whereas both customary international law and international treaties have not been updated about the answers yet. Some famous scholars have already provided their ideas about this issue and come to the conclusion of accepting the right to self-defense of victim states. However, those approaches should be examined and reexamined continuously. This thesis will attempt to unlock the above question and contribute more legal analysis relevant to the right of self-defense in case one state is attacked by cyber-attacks. The thesis is divided into three parts. The first part deals with questions like: What is self-defense? When and under what conditions, self-defense may legitimately take place. In the following, the focus of the discussion in the second part is defining the acceptable concept and the characteristics of cyber-attacks. Lastly, the third part wrestles with the most controversial topic that whether cyber-attacks can activate the right to self-defense under Article 51 UN Charter.