A motion guide device includes a ball nut attached onto a shaft and having helical grooves for receiving ball bearing elements, a ball circulating member is engaged with the ball nut and includes a ball circulating channel communicative with the helical groove of the ball nut, and includes an intermediate segment and two end segments, a ball coupler includes two flexible coupling members for engaging with the ball bearing elements, and the ball circulating member includes two opposite helical guide pathways for receiving the flexible coupling members of the ball coupler, and curved relative to the ball circulating member for smoothly guiding the flexible coupling members to move through the ball circulating member.