This invention provides an image pick-up optical lens assembly comprising, in order from an object side to an image side: a first lens element with positive refractive power having a convex object-side surface and a convex image-side surface; a second lens element with negative refractive power having a concave object-side surface and a convex image-side surface; a third lens element with positive refractive power having an object-side surface and a convex image-side surface, at least one of the surfaces thereof being aspheric; and a fourth lens element with negative refractive power having a convex object-side surface and a concave image-side surface, at least one of the surfaces thereof being aspheric. The image pick-up optical lens assembly further comprises an aperture stop disposed between an object and the first lens element. Such arrangement facilitates a significant reduction in size and sensitivity of the lens assembly while providing superb image quality with higher resolution.