An imaging system lens assembly comprising six lens elements (E1, E2, E3, E4, E5 and E6), the six lens elements (E1, E2, E3, E4, E5 and 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, E2, E3, E4, E5 and 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 in the imaging system lens assembly is six, the first lens element has positive refractive power, the object-side surface of the fifth lens element is concave in a paraxial region thereof, the image-side surface of the fifth 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 inflection point; wherein an axial distance between the image-side surface of the sixth lens element and an image surface is BL, an axial distance between the object-side surface of the first lens element and the image-side surface of the sixth lens element is TD, an f-number of the imaging system lens assembly is Fno, a curvature radius of the image-side surface of the fifth lens element is R10, a curvature radius of the object-side surface of the sixth lens element is R11, a curvature radius of the image-side surface of the sixth lens element is R12, a focal length of the imaging system lens assembly is f, a focal length of the first lens element is f1, a focal length of the second lens element is f2, a focal length of the third lens element is f3, a focal length of the fourth lens element is f4, and the following conditions are satisfied: 0.65 < BL / TD < 1.00; 1.35 < Fno < 2.30; − 65.00 < R 11 + R 12 / R 11 − R 12 < 1.80; − 0.90 < f 1 / f 3 < 2.00; − 12.00 < f / R 10 < − 1.10; and 0.05 < f 2 / f 4 < 0.95. wherein a total number of lens elements in the imaging system lens assembly is six, the first lens element has positive refractive power, the object-side surface of the fifth lens element is concave in a paraxial region thereof, the image-side surface of the fifth 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 inflection point; wherein an axial distance between the image-side surface of the sixth lens element and an image surface is BL, an axial distance between the object-side surface of the first lens element and the image-side surface of the sixth lens element is TD, an f-number of the imaging system lens assembly is Fno, a curvature radius of the image-side surface of the fifth lens element is R10, a curvature radius of the object-side surface of the sixth lens element is R11, a curvature radius of the image-side surface of the sixth lens element is R12, a focal length of the imaging system lens assembly is f, a focal length of the first lens element is f1, a focal length of the second lens element is f2, a focal length of the third lens element is f3, a focal length of the fourth lens element is f4, and the following conditions are satisfied: 0.65 < BL / TD < 1.00; 1.35 < Fno < 2.30; − 65.00 < R 11 + R 12 / R 11 − R 12 < 1.80; − 0.90 < f 1 / f 3 < 2.00; − 12.00 < f / R 10 < − 1.10; and 0.05 < f 2 / f 4 < 0.95.