Separation of powers is one of the important issues in the field of constitutional rights of the countries of the world. In the Islamic Republic of Iran and the United States of America, this issue has been emphasized in different ways in the constitution. The upcoming research deals with a comparative study between the separation of powers in these two countries. At the beginning, different models of the separation of powers are discussed, then the relations between the executive and legislative powers and the separation of powers beyond the two powers are examined. The result is that in America there is separation of powers of the type of presidential system, but none of the models of separation of powers have been implemented in Iran.