The optical design of a seeker, operating in two bands of infrared (MWIR-LWIR), is performed. The dual-band design has been developed on Zemax OpticStudio. Considering the optical parameters of commercially available dual-band IR detectors, the design of the IR imaging seeker is completed for the operation within the wavelength range of 3-10 mu m. The temperature dependence of the optical design is also examined for four different lens barrel materials, for the large temperature interval ranging from -40 degrees C to 80 degrees C. The defocusing effect caused by using different lens barrel is fixed by the passive-optical-athermalization technique. The field of view of the optical system is found to be 16 degrees. The modulation transfer function (MTF) value of each field is higher than 0.50 when the spatial frequency takes a value of 20.8 lp/mm. The results are analyzed and discussed in terms of spot diagram, MTF.