This invention provides a photographing optical system, in order from an object side to an image side comprising: a first lens element with positive refractive power having a convex object-side surface and concave image-side surface; a second lens element with negative refractive power; a third lens element with positive refractive power having a concave object-side surface and a convex image-side surface; and a fourth lens element with negative refractive power having a concave image-side surface and both the object-side and image-side surfaces thereof being aspheric; wherein the photographing optical system has four lens elements with refractive power. By such arrangement, the total track length and photosensitivity of the photographing optical system can be effectively reduced, and high image resolution can be obtained.