An image system lens assembly includes three lens groups including five lens elements. The three lens groups are, in order from an object side to an image side: first, second and third lens groups. The five lens elements are, in order from the object side to the image side: first, second, third, fourth and fifth lens elements. A zooming process is performed by changing axial distances between the three lens groups. The image system lens assembly has a long focal length end and a short focal length end. At least one surface of at least one lens element has an inflection point in an optically effective area thereof when the image system lens assembly is at the long focal length end and the short focal length end. The second lens group is moved relative to the first lens group along an optical axis in the zooming process.