An image pick-up optical system, in order from an object side to an image side, along an optical axis comprising: the first lens element with negative refractive power having a convex object-side surface and a concave image-side surface, the second lens element with positive refractive power, a bi-convex third lens element with positive refractive power, the fourth lens element with negative refractive power having a concave object-side surface, a plastic fifth lens element with positive refractive power having a convex object-side surface and at least one aspherical optical surface. Additionally, the image pick-up optical system satisfies conditions related to shorten the total length and to enhance the ability of wider field of view for use in compact cameras and mobile phones.