The k-ary n-cube has many good characteristics, and it has become one of the most commonly used interconnection network topologies in multiprocessor systems. The maintenance ability of system subnetworks plays an important role for the practical applications of the systems when failures occur in the interconnection network. In order to accurately measure the fault tolerance of subnetworks with arbitrary size in a k-ary n-cube, the reliability of k-ary (n-m)-cube subnetworks in a k-ary n-cube in the presence of failures was studied. When k was an odd integer and k was bigger than 2, the upper bound and lower bound on the probability that at least one k-ary (n-m)-cube subnetwork was fault-free in a k-ary n-cube were obtained under the probabilistic fault condition, and an approximate method for evaluating the reliability was proposed. Experimental results show that there is a gradual convergence between the upper bound and lower bound on the k-ary (n-m)-cube subnetwork reliability as the vertex reliability decreases, and the evaluation result obtained by the approximate method is relatively accurate when the vertex reliability is large.