An optical imaging system includes, in order from an object side to an image side, a first lens element with a positive refractive power, a second lens element, a third lens element, a fourth lens element with a positive refractive power having an aspheric surface and a fifth lens element with a negative refractive power. An image-side surface of the fifth lens element is concave, and at least one of two surfaces of the fifth lens element is aspheric. The relationship between a sum of thicknesses of all lens elements with refractive powers on an optical axis and a distance on the optical axis between an object-side surface of the first lens element and the image-side surface of the fifth lens element in the optical imaging system can effectively reduce the total length as well as the sensitivity of the optical imaging system while gaining superior resolution.