This Thesis was made to explain why the United Kingdom reopen its diplomatic relation with Iran in 2015 after the diplomatic cut ties between them in 2011. On November 29, 2011, The Brittish Embassy in Teheran closed due to the attack on the embassy building by Iranian protester's who against the sanction imposed by Britain after the IAEA report published. However, after four years, in 2015, the United Kingdom decided to reopen the Brittish Embassy in Teheran. Using the Foreign Policy theory, National Interest theory, and Rational Choice theory, the author analyzed any considerations that made Britain decided to reopen the diplomatic relation with Iran by calculating the advantages and disadvantages of the decision.