A foldable outside rearview mirror assembly includes a shaft arranged on an extending portion of the base, a frame having an and plate rotatably inserted into a root portion of the shaft, a mirror housing secured on the frame, a positioning device is mounted on the periphery of the shaft, a first washer is arranged at the upper portion of the positioning device, a second washer is secured on the upper end of the shaft, and a coil spring is arranged around the outer periphery of the shaft between the first and the second washers. The positioning device has a plate connected to the shaft in locking the rotation thereof, several convex portions provided on the same circumference of the plate, and several concaves formed at the frame end plate to insertingly engage with the convex portions. Thereby the frame of the outside rearview mirror is prevented from moving vertically and the chattering is not caused on the mirror surface at the time of the outside mirror foldably rotates.