Bacterial flagellar motor is an ion-driven supramolecular nanomachine embedded in the cell envelope. Rotor-stator interaction that couples to the specific ion translocation through the stator channel is the nature of torque generation. To produce fully functional motor, multiple stator complexes must be incorporated around the rotor at appropriate places. However, such stator assembly mechanism has not been investigated by the structural point of view. Here we describe stator assembly and activation mechanism revealed from the crystal structure of a motor component located in the periplasmic space, suggesting the dynamic conformational changes in the stator during its assembly-coupled activation.