The optical design and properties of fish-eye lens are discussed.A refractive/diffractive IR fisheye lens optical system is presented,whose operating wavelength range is 8 to12 μm,and full field of view is up to 240°. The structure of optical system is reversed telephoto lens, and this type of hyper-wide angle lens can balance the off-axis aberrations against the evenness of marginal image illuminance at its image plane very wel1. The results show that the system has the advantages of simplicity,compactness and long back working distance.Its imaging quality is very well,so that its modulated transferfunction value is higher than 0.5 when cutoff frequency at the 11 lines/mm.