In order to improve performance of uncooled thermal imager ,diffractive optical elements (DOEs) are applied in high relative aperture IR lenses. Compared with conventional design ,it enhances the ability of color correction ,improves the image quality and lenses transmission ,and reduces the elements and weight. DOEs provides fundamental basis for enhancing uncooled thermal imager performance. The paper compared four kinds IR lens in term of axial chromatic aberrations ,MTF ,transmission and MRTD.