Abstract: An optical system of roll-pitch infrared seeker with optical, mechanism integraled structure was presented. With the use of four reflective plane mirrors, the subassemblies of the optical system embed in the gimbal. The gimbal consisted of roll frame with N360 moving range and pitch frame with 90 moving range, so that the optical system could be adjusted to look over all of the front hemisphere field. The optical system design process was as follows: firstly, a reimaging system was set up, including front group, relay lens and rear group. As a high thermal gradient existed within the seeker, the front group must be athermal design alone, while the relay lens and rear group can be athermal design together. Then, the initial optical power distribution among lens can be resolved with the design method of mutual compensation of thermal dispersion. Finally, the optical system's aberrations were corrected by ZEMAX software. The analysis results show that when environment soaking temperature or gradient temperature varies from -40 ℃ to 80 ℃, the image preserves high quality and modulation transfer function (MTF) still approaches diffraction limit. The optical system achieves optical, structure and thermal integrated design.