An imaging optical lens system includes eight lens elements which are, in order from an object side to an image side along an optical path: a first lens element, a second lens element, a third lens element, a fourth lens element, a fifth lens element, a sixth lens element, a seventh lens element and an eighth lens element. The first lens element has positive refractive power. The second lens element has negative refractive power. The seventh lens element has an image-side surface being concave in a paraxial region thereof, and the image-side surface of the seventh lens element has at least one convex critical point in an off-axis region thereof.