An image capturing optical system includes, in order from an object side to an image side, a first lens element, a second lens element, a third lens element, a fourth lens element and a fifth lens element. The first lens element with positive refractive power has a convex object-side surface. The second lens element with negative refractive power has a concave object-side surface and a convex image-side surface. The third lens element has refractive power, wherein the surfaces of the third lens element are aspheric. The fourth lens element has positive refractive power, wherein the surfaces of the fourth lens element are aspheric. The fifth lens element with refractive power is made of plastic material, and has a concave image-side surface, wherein the surfaces of the fifth lens element are aspheric, and the fifth lens element has inflection points formed on at least one of the surfaces thereof.