This paper presents a non-scan design scheme to enhance delay fault testability of controllers. In this scheme, we utilize a given state transition graph (STG) to test delay faults in its synthesized controller. The original behavior of the STG is used during test application. For faults that cannot be detected by using the original behavior, we design an extra logic, called an invalid test state and transition generator, to make those faults detectable. Our scheme allows achieving short test application time and at-speed testing. We show the effectiveness of our method by experiments.