LEO Diaconus, or the Deacon, an historian of the tenth century, was born at Caloe, a town of Asia Minor, probably about A.D. 950. From his history we learn that he was studying at Constantinople in 966; that he was in Asia in 973 or 974, when Basilus I., Patriarch of Constantinople was deposed, and that in 981 he accompanied the Emperor Basilus II. on his expedition against the Bulgarians. We infer also, from some of the events he mentions, that he must have lived at least till 993. His works are,—Ἰστορία, Oratio ad Basilium Imperatorem, and Homilia in Michaelem Archangelum.
Of these, the two last exist only in manuscript. His History was first published at the expense of Count Nicolas Romanof, Chancellor of Russia, accompanied with a preface, a Latin version, notes, and engravings from ancient gems, by C. B. Hase, Paris, 1818. This edition, without the engravings, was reprinted in the Bonn edition of the Corpus Historiæ Byzantinæ, 1828.