  The concept of gcd-graphs is introduced by Klotz and Sander, which arises as
a generalization of unitary Cayley graphs. The gcd-graph $X_n (d_1,...,d_k)$
has vertices $0,1,...,n-1$, and two vertices $x$ and $y$ are adjacent iff
$\gcd(x-y,n)\in D = \{d_1,d_2,...,d_k\}$. These graphs are exactly the same as
circulant graphs with integral eigenvalues characterized by So. In this paper
we deal with the clique number of integral circulant graphs and investigate the
conjecture proposed in \cite{klotz07} that clique number divides the number of
vertices in the graph $X_n (D)$. We completely solve the problem of finding
clique number for integral circulant graphs with exactly one and two divisors.
For $k \geqslant 3$, we construct a family of counterexamples and disprove the
conjecture in this case.
