Advanced Si solar cells such as passivated emitter, hetero-junction and back contact solar cells are expected to use as space solar cells. In this paper, efficiency potential of crystalline Si space solar cells is analyzed by considering external radiative efficiency (ERE), voltage and fill factor losses. Crystalline Si space solar cells have efficiency potential of more than 26% at AM0 by realizing ERE of 20% from about 0.2% and normalized resistance of less than 0.05 from around 0.15. Non-radiative recombination and resistance losses in Si space solar cells are also discussed. Radiation degradation of Si space solar cells is also analyzed. Potential of advanced Si solar cells such as passivated emitter, hetero-junction and back contact solar cells for space applications is discussed from point view of radiation degradation.