After his abdication on 11 February 1866 Alexander John Cuza, both liberals and conservatives inclined most, to bring a prince from a foreign dynasty on the throne of Romania, which would be guaranteed political stability and social security.

Thus, bringing the country's perfected Carol of Hohenzollern-Sigmaringen, who was a cousin of the mother of Napoleon III and sister and the king of Prussia. Carol has accepted the proposal on 25 April 1866, the agreement of his father, the king of Prussia and Chancellor Bismarck to departure for Romania. Prince traveled to Turnu Severin using a false passport, the conflict between Austria and Prussia. He was accompanied on his journey of John C. Bratianu. On 10 May 1866, Charles I came to Bucharest, the Lord proclaimed by the Legislative Assembly Building in Metropolitan. One month before the coming of Charles I, was elected Legislative Assembly, which was transformed after his arrival in the Constituent Assembly. The Constituent Assembly was intended to discuss and vote on a draft Constitution. The new fundamental law was promulgated by the gentleman from July 1, 1866.

Constitution of 1866 is the first actual constitution of Romania, adopted the model constitution of Belgium in 1831. Constitution was adopted by King Carol I, the longest-running constitution (1866-1923) and the most important achievement of the regime of Charles I. It was promulgated by the gentleman from July 1, 1866.

It turns Romania into hereditary constitutional monarchy (in the direct descending line and his Highness Prince legitimate Carol I of Hohenzollern-Sigmaringen, the man in man by order of primogeniture and perpetual women exclusiveness – Article 82) and provide principles democracy and separation of powers, accountability ministerial rights and freedoms, but also maintained the consistory vote (people could vote on a sum of money paid, so that those who voted were often large landowners, mostly conservative).

It was the first constitution drafted without foreign competition, and without external approval, making the event an act of independence, irrespective of Ottoman suzerainty and guarantee collection of the seven major powers, while providing the basis for the evolution of the modern Romanian state and democratic.

Constitution of 1866 and ceased applicability date of entry into force of the Constitution of 1923.