In this paper, we give a necessary and sufficient condition for the integrality of Cayley graphs over the dihedral group D n = a, b | a n = b 2 = 1, bab = a −1 . Moreover, we also obtain some simple sufficient conditions for the integrality of Cayley graphs over D n in terms of the Boolean algebra of a , from which we find infinite classes of integral Cayley graphs over D n . In particular, we completely determine all integral Cayley graphs over the dihedral group D p for a prime p.