The present invention relates to an electrical connector assembly and a method to assemble the same, wherein the electrical connector assembly comprises a plug connector that is adapted to be mated with a corresponding counter connector 80. The plug connector comprises a connector housing 20 having at least one guiding means and a locking device 10 being arranged movable relative to the connector housing 20, wherein said locking device 10 has at least one guiding member and at least one flexible arm 12, wherein the flexible arm 12 is adapted to be flexed during mating the plug connector with the corresponding counter connector 80 such that the locking device 20 can be moved into a locked position, wherein the guiding means and the guiding member are adapted to guide the locking device 10 from a first position to a locked position, so that the flexible arm 12 is reflexed when the locking device 10 is in the locked position.