An optical imaging system comprising six lens elements (E1-E6), the six lens elements (E1-E6) being, 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 and a sixth lens element, and each of the six lens elements (E1-E6) 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 (E1-E6) in the optical imaging system is six, the optical imaging system further comprises an aperture stop disposed between the second lens element and the third lens element, the first lens element has negative refractive power, the object-side surface of the first lens element is concave in a paraxial region thereof, the object-side surface of the first lens element has at leat one critical point in an off-axis region thereof, the second lens element has positive refractive power, the object-side surface of the second lens element is convex in a paraxial region thereof, the image-side surface of the second lens element is concave in a paraxial region thereof, the third lens element has positive refractive power, the object-side surface of the third lens element is convex in a paraxial region thereof, the image-side surface of the third lens element is convex in a paraxial region thereof, the fourth lens element has negative refractive power, the fifth lens element has positive refractive power, the sixth lens element has negative refractive power, the object-side surface of the sixth lens element is convex in a paraxial region thereof, the image-side surface of the sixth lens element is concave in a paraxial region thereof, and the image-side surface of the sixth lens element has at least one critical point in an off-axis region thereof; wherein an Abbe number of the sixth lens element is V6, a focal length of the optical imaging system is f, a curvature radius of the object-side surface of the fourth lens element is R7, a curvature radius of the image-side surface of the fourth lens element is R8, a central thickness of the first element is CT1, a central thickness of the second lens element is CT2, a central thickness of the third lens element is CT3, a central thickness of the fourth lens element is CT4, an axial distance between the first lens element and the second lens element is T12, an axial distance between the second lens element and the third lens element is T23, an axial distance between the fourth lens element and the fifth lens element is T45, and the following conditions are satisfied: 10.0 < V6 < 44.0; − 1.6 < f / R7 + f / R8 < 0.10; 0 < CT4 / T45 < 1.6; 0.35 < CT1 / T12 < 1.5; and 2.0 < CT2 + CT3 / T23 < 5.8. wherein a total number of lens elements (E1-E6) in the optical imaging system is six, the optical imaging system further comprises an aperture stop disposed between the second lens element and the third lens element, the first lens element has negative refractive power, the object-side surface of the first lens element is concave in a paraxial region thereof, the object-side surface of the first lens element has at leat one critical point in an off-axis region thereof, the second lens element has positive refractive power, the object-side surface of the second lens element is convex in a paraxial region thereof, the image-side surface of the second lens element is concave in a paraxial region thereof, the third lens element has positive refractive power, the object-side surface of the third lens element is convex in a paraxial region thereof, the image-side surface of the third lens element is convex in a paraxial region thereof, the fourth lens element has negative refractive power, the fifth lens element has positive refractive power, the sixth lens element has negative refractive power, the object-side surface of the sixth lens element is convex in a paraxial region thereof, the image-side surface of the sixth lens element is concave in a paraxial region thereof, and the image-side surface of the sixth lens element has at least one critical point in an off-axis region thereof; wherein an Abbe number of the sixth lens element is V6, a focal length of the optical imaging system is f, a curvature radius of the object-side surface of the fourth lens element is R7, a curvature radius of the image-side surface of the fourth lens element is R8, a central thickness of the first element is CT1, a central thickness of the second lens element is CT2, a central thickness of the third lens element is CT3, a central thickness of the fourth lens element is CT4, an axial distance between the first lens element and the second lens element is T12, an axial distance between the second lens element and the third lens element is T23, an axial distance between the fourth lens element and the fifth lens element is T45, and the following conditions are satisfied: 10.0 < V6 < 44.0; − 1.6 < f / R7 + f / R8 < 0.10; 0 < CT4 / T45 < 1.6; 0.35 < CT1 / T12 < 1.5; and 2.0 < CT2 + CT3 / T23 < 5.8.