An image capturing optical system comprising four lens elements (E1, E2, E3, E4), the four lens elements (E1, E2, E3, E4) being, in order from an object side to an image side along an optical path, a first lens element (E1), a second lens element (E2), a third lens element (E3) and a fourth lens element (E4), and each of the four lens elements (E1, E2, E3, E4) having an object-side surface facing toward the object side and an image-side surface facing toward the image side; wherein a total number of lens elements of the image capturing optical system is four, the first lens element (E1) has negative refractive power, the image-side surface of the first lens element (E1) is concave in a paraxial region thereof, the second lens element (E2) has positive refractive power, the third lens element (E3) has positive refractive power, the object-side surface of the third lens element (E3) is concave in a paraxial region thereof, the image-side surface of the third lens element (E3) is convex in a paraxial region thereof, the fourth lens element (E4) has negative refractive power, the object-side surface of the fourth lens element (E4) is convex in a paraxial region thereof, the image-side surface of the fourth lens element (E4) is concave in a paraxial region thereof, there is an air gap in a paraxial region between the second lens element (E2) and the third lens element (E3), and characterized in that the image capturing optical system further comprises an aperture stop (ST) located between the second lens element (E2) and the third lens element (E3); wherein a curvature radius of the object-side surface of the second lens element (E2) is R3, a curvature radius of the image-side surface of the second lens element (E2) is R4, a focal length of the image capturing optical system is f, a central thickness of the second lens element (E2) is CT2, a central thickness of the third lens element (E3) is CT3, and the following conditions are satisfied: − 1.65 < R 3 + R 4 / R 3 − R 4 ; and 0.60 < f / CT 2 + CT 3 < 2.50 . wherein a total number of lens elements of the image capturing optical system is four, the first lens element (E1) has negative refractive power, the image-side surface of the first lens element (E1) is concave in a paraxial region thereof, the second lens element (E2) has positive refractive power, the third lens element (E3) has positive refractive power, the object-side surface of the third lens element (E3) is concave in a paraxial region thereof, the image-side surface of the third lens element (E3) is convex in a paraxial region thereof, the fourth lens element (E4) has negative refractive power, the object-side surface of the fourth lens element (E4) is convex in a paraxial region thereof, the image-side surface of the fourth lens element (E4) is concave in a paraxial region thereof, there is an air gap in a paraxial region between the second lens element (E2) and the third lens element (E3), and characterized in that the image capturing optical system further comprises an aperture stop (ST) located between the second lens element (E2) and the third lens element (E3); wherein a curvature radius of the object-side surface of the second lens element (E2) is R3, a curvature radius of the image-side surface of the second lens element (E2) is R4, a focal length of the image capturing optical system is f, a central thickness of the second lens element (E2) is CT2, a central thickness of the third lens element (E3) is CT3, and the following conditions are satisfied: − 1.65 < R 3 + R 4 / R 3 − R 4 ; and 0.60 < f / CT 2 + CT 3 < 2.50 . <EOS>