A clutch failure diagnostic method for a dual clutch transmission (DCT), may include a synchronous clutch confirming step of confirming whether a clutch rotating in synchronization with an engine speed may be present after a cranking of an engine, when the clutch rotating in the synchronization with the engine speed may be determined to be present, a gear joining attempting step of attempting to join shift gears connected to the clutch rotating in the synchronization with the engine speed, and a failure determining step of determining that a corresponding clutch fails when the gears may be not joined, as a performance result of the gear joining attempting step.