At Iasi he started his Arhiva historica a Romaniei (1865-1867), in which a large number of old documents in Slavonic and Romanian were published for the first time. In 1870 he inaugurated "Columna lui Traian", the best philological review of the time in Romania. In his Cuvente den Batrāni (2 vols., 1878-1881) he was the first to contribute to the history of apocryphal literature in Romania.
His Historia entice a Romanilor (1875), though incomplete, marks the beginning of critical investigation into the history of Romania. Hasdeu edited the ancient Psalter of Coresi of 1577 (Psaltirea lui Coresi, 1881).
His Etymologicum magnum Romaniae (1886) is the beginning of an encyclopaedic dictionary of the Romanian language, though never finished the letter B. In 1876 he was appointed director of the state archives in Bucharest and in 1878 professor of philology at the University of Bucharest. His works, which include one drama, Rasvan si Vidra, bear the impress of great originality of thought, and the author is often carried away by his profound erudition and vast imagination.
Hasdeu was a keen politician. After the death of his only child, Julia, in 1888, he became a mystic anti a strong believer in spiritism. He died at Cāmpina on the September 7, 1907.