  A Cayley (di)graph $Cay(G,S)$ of a group $G$ with respect to a subset $S$ of
$G$ is called normal if the right regular representation of $G$ is a normal
subgroup in the full automorphism group of $Cay(G,S)$, and is called a
CI-(di)graph if for every $T\subseteq G$, $Cay(G,S)\cong Cay(G,T)$ implies that
there is $\sigma\in Aut(G)$ such that $S^\sigma=T$. We call a group $G$ a
NDCI-group if all normal Cayley digraphs of $G$ are CI-digraphs, and a
NCI-group if all normal Cayley graphs of $G$ are CI-graphs, respectively. In
this paper, we prove that a cyclic group of order $n$ is a NDCI-group if and
only if $8\nmid n$, and is a NCI-group if and only if either $n=8$ or $8\nmid
n$.
