The second half of the nineteenth century is one of the most important periods for the evolution of political thought in Persia. Notable intellectuals started to develop a critical perception of the social, economic and political issues, giving life to a new current of thought which became known as the constitutionalist movement of Iran. Mirzā Aqā Khān Kermāni (1853-1896) is one of the most important thinkers of this time who, in numerous books and articles, puts forward constitutionalist and nationalist ideas for the first time in Iranian contemporary history. This article will examine his political thought, highlighting how Kermāni, after being in contact with the liberal and constitutional ideas of western thinkers and also after a new revision of his own Persian history and philosophy, was able to work out a new way of thinking which contributed to creating the ideological basis for a modern concept of Nationalism in Iran and for the first attempt at secularization of Iranian society. This article shows how the Iranian constitutional revolution of 1906 (mashrūṭeh) was in fact partially the fruit of ideas of this first generation of secular thinkers such as Kermāni, who were able to spread a new civil concept of the state which separated religion from politics, by introducing the rule of law and limiting the absolute power of the monarch.