Electrophysiological studies have shown that cortico-spinal excitability increases during the motor preparation period in reaction time (RT) paradigms. However, there is a line of contradictory evidence with transcranial magnetic stimulation (TMS) showing that its excitability is reduced during the preparation period. In these studies, the subjects can predict the TMS timings. Thus we investigated how the predictability of TMS timing affects cortico-spinal excitability. A single-pulse TMS was delivered to the hand section of the left motor cortex while seven right-handed subjects relaxed their hands in a flexed position. We prepared three conditions: (i) in the semi-PREDICTABLE condition, two visual stimuli at 500 ms interval were presented and the TMS was delivered either 0, 125, 250, 375, or 500 ms after the first stimulus; (ii) in the PREDICTABLE condition, the TMS was provided only at 500 ms after the first stimulus; (iii) in the UNPREDICTABLE condition, no visual cue preceded the TMS. We recorded motor evoked potentials (MEPs) from the wrist flexor and extensor muscles. We found a significant reduction of MEP amplitude in the flexor muscles in both the PREDICTABLE and semi-PREDICTABLE conditions, but not in the UNPREDICTABLE condition. These results showed that the predictability of TMS per se, without the preparation of motor outputs, can reduce cortico-spinal excitability.