A four-piece infrared single wavelength lens system includes, in order from the object side to the image side: a stop, a first lens element with a refractive power having an object-side surface being convex near an optical axis, a second lens element with a positive refractive power, a third lens element with a positive refractive power having an object-side surface being concave near the optical axis and an image-side surface being convex near the optical axis, a fourth lens element with a negative refractive power having an object-side surface being convex near the optical axis and an image-side surface being concave near the optical axis. Such arrangements can provide a four-piece infrared single wavelength lens system which has a wide field of view, large stop, short length and less distortion.