We report a new three-dimensional ab initio potential energy surface of the first excited singlet (1(1)A ") state of HCN. More than one thousand potential points have been calculated at the multi-reference configuration interaction level employing a large basis set. Variational calculations of the predissociative resonances of both HCN and DCN are performed on the three-dimensional spline fitted potential energy surface, which yield both positions and widths. Fairly good agreement with experiment confirms the accuracy of our new potential energy. These resonances provide valuable information on the photodissociation dynamics of HCN/DCN in the 1(1)A " state.