Abstract: A padlock and key assembly includes a padlock and a cylindrical key. The padlock includes a lock body, and a key plug which is locked in the lock body by transversely movable tumbler units and vertically movable tumbler plate units. The key includes annular grooves formed in its outer surface, and push blocks extending radially from the key. When the key is inserted into the key plug, the annular grooves engage with the tumbler pin units and the push blocks engage with the tumbler plate units so that the key plug is unlocked from the lock body, thereby enabling the key plug to rotate with the key.