Optical design of a small reflecting telescope for use in a 1U CubeSat mission is reported in this study. A Ritchey-Chretien with field correction lens type telescope for earth observation techniques is adopted in this design. The primary mirror and secondary mirror are circular apertures with 70-mm and 21-mm in diameter. The effective focal length is 390-mm operated at 600-km altitude. A commercial 2560 × 2160 CCD image sensor CIS2521 with a pixel size of 6.5 μm is applied, which capture a 25km swath area. The ground resolution is better than 10m for CubeSat application. The MTF is expected to be about 0.3 at Nyquist frequency at 77lp/mm. The tolerance analysis is performed for further understanding on fabrication and assembly errors. In order to reduce the telescope size, the optical system uses a reflective optical system, and the circuit board is arranged behind the system and above. The telescope envelope size(all contained) is less than 96mm × 90mm × 90mm and the weight is approximately 600g. At present, the aspherical primary mirror has completed the processing and assembly. © 2019 SPIE.