This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDA's project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511-3514). Newly developed 2 x 2 cm(2) Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AM0, 28 degreesC after 1 MeV electron irradiation up to 1 x 10(15) e/cm(2) and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p-n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells. (C) 2001 Elsevier Science B.V. All rights reserved.