The invention provides an imaging lens which sequentially includes a first lens with positive focal power, a second lens with negative focal power, a third lens with positive focal power, and a fourthlens with negative focal power from an object side to an image side. The imaging lens satisfies conditional expressions as follows: T/F is larger than 1 and smaller than 1.29; R3/F is larger than 15;R7 is larger than 0; and R6 is smaller than 0, wherein T is the whole length of the imaging lens, F is the effective focal length of the imaging lens, and R3, R6 and R7 are respectively the curvatureradius of the object side surface of the second lens, the curvature radius of the image side surface of the third lens, and the curvature radius of the object side surface of the fourth lens. The conditional expression that T/F is larger than 1 and smaller than 1.29 is used for shortening the whole length of the imaging lens. The conditional expression that R3/F is larger than 15 is used for correcting monochromatic aberration. The conditional expressions that R7 is larger than 0 and R6 is smaller than 0 are used for correcting aberration and field curvature. Therefore, the resolution is improved while the size is reduced.