  In 1988, I.Beck showed that the chromatic number of G(Zn) is equal to its
clique number.In 2004, S.Akbari and A.Mohammadian proved that the edge
chromatic number of G(Zn) is equal to its maximum degree,in 2008,
J.Skowronek-kaziow give formulas calculating the clique number and the maximum
degree of G(Zn), but he have a error about clique number of G(Zn), we consider
the zero-divisor graph G(Zn) of the ring Zn.we give formulas calculating the
clique number of G(Zn).We present a constructed method to calculate the clique
number.
