  The aim of this article is to give a criterion, generalizing the criterion
introduced by Priddy for algebras, to verify that an operad is Koszul. We
define the notion of a Poincare-Birkhoff-Witt basis in the context of operads.
Then we show that an operad having a Poincare-Birkhoff-Witt basis is Koszul.
Besides, we obtain that the Koszul dual operad has also a
Poincare-Birkhoff-Witt basis. We check that the classical examples of Koszul
operads (commutative, associative, Lie) have a Poincare-Birkhoff-Witt basis.
