Locking of the metacarpophalangeal (MP) joint of the fingers, though reported infrequently, is not rare in the literature. We will report two rare cases of the MP joint of the thumb locked in 90 degrees of flexion (vertical locking). The first case is a 21-year-old man, punched on his right thumb by his friend, who arrived with his thumb fixed in a flexed position. The X-ray images of the right thumb showed the proximal phalanx subluxation in the palmer side in a vertical position. The second case is a 35-year-old woman with her right thumb accidentally caught in the chain of a key-holder. The locking was easily reduced without anaesthesia in both cases. We assume the mechanism was that the flexion force on the MP joint led to subluxation and the locking occurred due to the tension of the collateral ligament caused by the volar prominence of the radial condyle.