A board-to-board connector assembly includes a receptacle connector and a plug connector mated with each other. 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 and having a base portion. A top of the base portion is bent toward one side and then extends downward to form a propping portion defining a first fixing structure thereon. The plug connector has a plug housing, a plurality of second terminals disposed in the plug housing and contacting the corresponding first terminals electrically, and at least one second fixing member mounted to the plug housing and having a base board. At least one end of the base board extends downward to form a connecting portion defining a second fixing structure thereon which can be buckled with the first fixing structure.