A board-to-board connector assembly includes a receptacle connector and a plug connector. The receptacle connector has a receptacle housing, a plurality of first terminals disposed in the receptacle housing, and at least one first fixing member mounted to the receptacle housing. The first fixing member has a first base portion, an elastic portion bending toward one side and then extending downward from a top of the first base portion. An outside portion of the elastic portion defines a fixing cavity opposite to the first base portion. The plug connector has a plug housing mated with the receptacle housing, a plurality of second terminals disposed in the plug housing and contacting the corresponding first terminals, and at least one second fixing member mounted in the plug housing and defining at least one fixing portion which is buckled into the fixing cavity of the corresponding first fixing member.