Let ϕ be Euler's phi function. We prove that a vertex-transitive graph of order n, with gcd(n, ϕ(n)) = 1, is isomorphic to a circulant graph of order n if and only if Aut( ) contains a transitive solvable subgroup. As a corollary, we prove that every vertex-transitive graph of order n is isomorphic to a circulant graph of order n if and only if for every such , Aut( ) contains a transitive solvable subgroup and n = 4, 6, or gcd(n, ϕ(n)) = 1.