An image capturing optical system includes, in order from an object side to an image side: the first lens element with positive refractive power having a convex object-side surface, the second lens element with positive refractive power having a concave object-side surface and a convex image-side surface, the third lens element with positive refractive power having a concave object-side surface and a convex image-side surface, wherein the object-side surface and the image-side surface of the third lens element are aspheric. By such arrangement, the total track length and the photosensitivity of the image capturing optical system can be effectively reduced while retaining high image quality. The image capturing optical system can also be applied to an infrared optical system.