We say that a polynomial differential systemẋ = P (x, y),ẏ = Q(x, y) having the origin as a singular point is Z 2-symmetric if P (−x, −y) = −P (x, y) and Q(−x, −y) = −Q(x, y). It is known that there are nilpotent centers having a local analytic first integral, and others which only have a C ∞ first integral. But up to know there are no characterized these two kinks of nilpotent centers. Here we prove that the origin of any Z 2-symmetric is a nilpotent center if, and only if, there is a local analytic first integral of the form H(x, y) = y 2 + • • •, where the dots denote terms of degree higher than two. 2010 Mathematics Subject classification. 34C05, 34A34, 34C14. Keywords and phrases. Z 2-symmetric differential systems, center problem, nilpotent singularity.