A locking system for mechanical joining of rectangular floorboards having a body and long and short edges. Said locking system including a tongue and a groove for vertical locking of two joint short edges and for horizontal joining a locking groove formed in the underside of a first one of the short joint edges, and a portion projecting from the second short joint edge having a locking element cooperating with the locking groove. The tongue is arranged on the second short edge. The groove is arranged on the first short edge. The projecting portion is integrally formed with the board, by working of the body of the board.