In this paper we consider the real topological K -groups of a toric manifold M , which turns out to be closely related to the topology of the small cover M R , the fixed points under the canonical conjugation on M . Following the work of Bahri and Bendersky [2], we give an explicit formula for the KO -groups of toric manifolds, and then we characterize the two extreme classes of toric manifolds according to the two A(1) modules shown in [2].