Abstract: A mannequin includes a support frame and a horizontal support assembly which includes an upper horizontal support unit, a middle horizontal support unit and a lower horizontal support unit. Each of the horizontal support units includes a generally horizontal middle tube mounted movably on the support frame, two generally horizontal side tubes mounted respectively and movably within two end portions of the horizontal middle tube, and an adjustment unit installed on the horizontal middle tube and actuatable so as to move the horizontal side tubes on the horizontal middle tube. Each of the horizontal side tubes includes a rack secured to an inner end portion thereof. The adjustment unit is provided with a fixed pinion that engages the racks of the horizontal side tubes so that rotation of the adjustment unit moves the horizontal side tubes relative to the horizontal middle tube. A chest-outline simulating plate is supported on the top end portion of the support frame.