A matrix board to board connector assembly includes a first connector and a second connector. A first locking piece is provided and fixed on the first connector. The first locking piece has a locking portion bend from a lower edge thereof and extending sideward and outward. A second locking piece is provided with an elastic portion and fixed on the second connector. The elastic portion is configured to provide a spring end. The spring end is firstly protruded and then concaved at its free end. During mating of the first and the second connectors, the spring end is firstly deflected and then elastically recovered to hit on the locking portion with a click to insure that the first and the second connectors have been properly mated.