A zoom lens system includes, in order from an object side to an image side along an optical axis thereof, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power. When zooming from a wide-angle end to a telephoto end, both the first and third lens groups move to the object side, and the second lens group first moves to the image side and then moves to the object side, whereby a first variable distance between the first and second lens groups is increased and a second variable distance between the second and third lens groups is decreased. The fourth lens group is also movable along the optical axis, so as to change a third variable distance between the third and fourth lens groups. Each of the third and fourth lens groups includes at least one aspheric surface. The zoom lens system provides a high zoom ratio and a high image resolution, and is short in overall length and small in size.