Let π be an irreducible cuspidal automorphic representation of a quasi-split unitary group U n defined over a number field F . Under the assumption that π has a generic global Arthur parameter, we establish the non-vanishing of the central value of L-functions, L( 1 2 , π × χ), with a certain automorphic character χ of U 1 , for the case of n = 2, 3, 4, and for the general n ≥ 5 by assuming Conjecture 1.4 on certain refined properties of global Arthur packets. In consequence, we obtain some simultaneous non-vanishing results for the central L-values by means of the theory of endoscopy.