The present invention is to provide a structure of key part having hideable lock cylinder, which comprises a base unit having an end able to be fixed on an object body; a lock cylinder unit mounted at the other end of the base unit and provided with a cylinder column at an end thereof; and a key unit provided with a locking piece at a side edge of an end thereof, having the other end pivoted on the lock cylinder unit at a position near the cylinder column, and having a groove axially provided thereon for accommodating the cylinder column; whereby the cylinder column can be inserted into a lock adjusting slot of a handcuff, or can be hidden in the key unit while not in use. Thus, a user needs not to worry that pocket of his or her clothing will be hooked and damaged by the cylinder column.