A mounting portion structure of a propeller shaft center bearing in a Rear-Wheel Drive (RWD) vehicle where the engine power is transmitted to the rear wheels by using the propeller shaft includes a tunnel formed at the center of a center floor panel along the longitudinal direction of the vehicle. A mounting bracket is equipped at the tunnel and is mounted with the center bearing. A pocket groove is formed at the mounting bracket for rendering a closed box with the tunnel, and a pipe nut is equipped at the pocket groove.