Changing the letters happens because of different reasons such as changing of the government, changing of the writing tools, the advent of technology, the defects of letters, the influence of the new culture, and changing of the scientific language. All these reasons either alone or together can be the source of changing the letters in a country or area. If the issue of changing the letters is considered apart from changing of the government, it cannot certainly have a definite time since its causative factors are not momentary events. The invention of alphabets and its function is one of the inventions which is gradually altered or completed. Changing the letters in Iran is not an exception, and it has got a long history. In the following study, the author intends to present the history of the idea of changing the letters in Iran. In this regard, he briefly looks into Iranian letters before Islam, and the way Kufiletters transformed into Pahlavi ones. Then, he starts talking about the fields which lead to the rising the idea of changing the letters in recent centuries; and mentions some of the effective elements. Some of them are the arrival of publishing industry in Iran, changing of Iran social state, Iran’s defeats in front of Russia, the reformations in Iran, the effect of Ottoman and Ottoman Iranians on Iran. The writer explains every one of these reasons. In another part of the article, he examines the issues of changing the letters abroad, changing of Arabic characters in some countries, and the new alphabets in Turkey. He continues his discussion by talking over how the idea of changing the letters arrives in Iran, different debates about this issue, and the reasons these activities have no result. Finally, he points out the decline of this subject by the advent of Islamic revolution and the final changing of letters in Iran.