Lightweight mirrors of SiC (Silicon Carbide) are currently used for numerous space telescopes. The most favorable characteristics of the material are high specific stiffness, high thermal conductivity and low thermal expansion (CTE). For large-scale SiC mirrors, however, the machining cost is very high using traditional polishing process. In this study, a large-scale SiC spherical mirror (φ360mm) was designed as reducing-weight structure with many open-back triangular cells. The ELID mirror grinding technique was conducted to finish the mirror. A unique jig composed of many hydrostatic actuators was developed for reducing the deformation resulted from the grinding force. By using an on-machine measurement system, the form error of the finished surface was measured. The surface roughness of the mirror was also examined with a mobile AFM. As result, a high quality mirror surface, which has form accuracy 0.8µm P-V and roughness of Ra 7.8nm, was achieved. This paper presents the experimental procedure and results.