An ocular optical system for imaging of imaging rays entering an eye of an observer via the ocular optical system from a display screen is provided. A side facing towards the eye is an eye-side, and a side facing towards the display screen is a display-side. The ocular optical system includes a first lens element, a second lens element, and a third lens element from the eye-side to the display-side in order along an optical axis. The first lens element, the second lens element, and the third lens element each include an eye-side surface and a display-side surface. Lens elements having refracting power of the ocular optical system are only the first lens element, the second lens element, and the third lens element.