The disclosure provides an optical imaging system. According to one embodiment, the optical imaging system, in order from an object side to an image side, includes a first lens element and a second lens element with refractive power. At least one of the first lens element and the second lens element has positive refractive power. The focal length of the optical imaging system is f; the diameter of the entrance pupil of the optical imaging system is HEP; the distance from an object-side surface of the first lens element to an image plane is HOS; the distance from the object-side surface of the first lens element to the image-side surface of the second lens element is InTL; the length of outline curve from an axial point on any surface of any one of the two lens elements to a coordinate point of vertical height with a distance of a half of the entrance pupil diameter from the optical axis on the surface along an outline of the surface is denoted as ARE. The following relations are satisfied: 1.2<=f/HEP<=3.5, 0.5<=HOS/f<=3.0, 0