Let $G$ be a finite subgroup of SU(2) or SO(3). McKay correspondence describes the classical geometry of G-Hilb (the Hilbert scheme of $G$-clusters in $C^2$ or $C^3$) in terms of the representation theory of $G$. We determine the Gromov-Witten invariants of $G$-Hilb and describe it in terms of a root system canonically associated to $G$. This in particular describes the quantum cohomology of $G$-Hilb in terms of the associated root system.