This invention provides an imaging lens system in order from an object side to an image side comprising five lens elements with refractive power: a first lens element with positive refractive power having a convex object-side surface; a second lens element with negative refractive power; a plastic third lens element having at least one of the object-side and image-side surfaces thereof being aspheric; a plastic fourth lens element having a concave image side surface, at least one of the object-side and image-side surfaces thereof being aspheric; and a plastic fifth lens element having a convex object side surface and a convex image-side surface, at least one of the object-side and image-side surfaces thereof being aspheric. By such arrangement, especially with the fourth lens element having the concave image side surface, the imaging lens system can correct peripheral rays while further improving the image resolution thereof.