A lens assembly for an image sensor comprises an aperture, a first lens, a second lens and a third lens, which are arranged sequentially from the object side. The first lens is a planoconvex lens or a positive meniscus lens whose convex surface faces the object side; the second lens is a negative meniscus lens with a convex surface facing the image side and at least has an aspherical surface; and the third lens is a positive meniscus lens with a convex surface facing the object side and, at least, has an aspherical surface. The sensitivity of decentration and the aberration-correction effect of the whole lens assembly in accordance with the present invention can be effectively improved.