In this paper, we give an explicit representation and enumeration for negacyclic codes of length 2 k n over the local non-principal ideal ring R = Z 4 + uZ 4 (u 2 = 0), where k, n are arbitrary positive integers and n is odd. In particular, we present all distinct negacyclic codes of length 2 k over R precisely. Moreover, we provide an exact mass formula for the number of negacyclic codes of length 2 k n over R and correct several mistakes in some literatures.