This paper proposes a novel lightweight robot arm for astronautical development by using honeycomb structure. Honeycomb structure is one of typical lightweight material and already used in astronoutical field mainly for large panels. Besides, Carbon-Fiber Reinforced Plastic (CFRP) is already known to be a lightweight and strong material. In this study, we propose to apply honeycomb structure with CFRP to realize lightweight and compact arm. A prototype using CFRP plates and aluminum honeycomb is produced and its mechanical property is investigated experimentally. The effect of cross-sectional shape and size of honeycomb core onto the mechanical property is also investigated. Moreover, theoretical analysis using FEM is under way.