This invention provides an imaging lens system in order from an object side to an image side comprising: a first lens element with positive refractive power; a plastic second lens element with negative refractive power having a concave object-side surface and a convex image-side surface, both surfaces thereof being aspheric; and a plastic third lens element with negative refractive power having a convex object-side surface and a concave image-side surface, both surfaces thereof being aspheric, and at least one inflection point is formed on at least one of the object-side and image-side surfaces thereof. Additionally, the central thickness of the second lens element is controlled favorably for the efficient spatial arrangement of the lens assembly and the simpler individual lens production while maintaining suitable thickness of the second lens element, thereby assuring image quality and improving yield rate of the product.