A telephoto zoom lens include a first, a second, a third and a fourth lens groups sequentially arranged along an optical axis and from an object side to an image side. The refractive powers of the four lens groups are positive, negative, positive and positive, respectively. The first lens group is fixed at a first predetermined position. The second lens group is movable along the optical axis depending on the variation of the magnifying power of the zoom lens. The third lens group is fixed at a second predetermined position. The fourth lens group is movable along the optical axis to keep an image plane generated by the zoom lens to project accurately onto an image sensor. Hence, the first and the third lens groups are in resting state, and the second and the fourth lens groups are movable during zoom-in or zoom-out operation of the zoom lens.