The present invention discloses an image system lens assembly and an image capturing device. The image lens assembly includes, in order from an object side to an image side, 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. The first lens element has positive refractive power. The second lens element has positive refractive power. The third lens element has refractive power. The fifth lens element has a concave object-side surface, wherein both of the object-side and image-side surfaces of the fifth lens element are aspheric. The sixth lens element has a concave image-side surface, wherein the image-side surface thereof has at least one inflection point, and both of the object-side and image-side surfaces of the sixth lens element are aspheric. The image system lens assembly has a total of six lens elements with refractive power. When the specified conditions are satisfied, the excessive surrounding curvature of the lenses can be avoided, and further, the aberration of an off-axis view field is reduced.