A zoom-lens structure for a camera is disclosed which comprises at least an outer cylinder, an inner cylinder, a fastening ring and a lens mount assembly. The outer cylinder has a plurality of straight grooves for engaging with a plurality of projected keys, respectively, provided on a fastening ring so as to allow the fastening ring to be movably mounted outside the inner cylinder. The lens mount assembly includes a first lens mount and a second lens mount, and the fastening ring has at least two first positioning pieces to be engaged with positioning grooves provided on the second lens mount so as to limit the second lens mount from rotating. At least two second positioning pieces are provided on the first lens mount which are also engaged with the positioning grooves and are thus controlled by the second lens mount from being able to rotate. This arrangement controls the entire lens mount assembly to move back and forth along an optical axis to provide a zoom lens function. In a preferred embodiment, the inner surface of the inner cylinder is furnished with a plurality of curved grooves for receiving corresponding rolling balls, respectively, provided on outer surfaces of the first lens mount and the second lens mount. The zoom-lens structure so disclosed allows the focal length to be varied with only three cylinders having relatively small diameters without affecting the retracting length of the lens assembly.