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Silsesquioxane (RSiO 3/2) has been researched as a potential atomic oxygen (AO) protective coating material for spacecraft applications. To this end, the present study focuses on the development of photocurable silsesquioxane (SQ series, manufactured by Toagosei Co., Ltd.) coating on polyimide base films. Photocurable silsesquioxanes "SQ series" are a new type of organic-inorganic hybrid materials that feature photoinitiated polymerizable groups (organic units) introduced into silsesquioxane frameworks (inorganic units) as multifunctional groups at an intramolecular level. This coating has the following excellent properties for use as a protective coating for space materials: 1) high AO tolerance, 2) high adhesion to substrate material (polyimide), 3) high transparency, 4) low outgassing and low re-condensability, and 5) ease of handling. In this study, we selected two types of space-qualified polyimide films, both of which were 25 µm thick and aluminized on the rear. We adopted roll-to-roll wet coating technology for this product with a width of 1 m. The SQ-coated films thus obtained were evaluated for their resistance to space environmental factors such as AO, electron beam, and ultraviolet irradiations; thermal cycling; and high-temperature vacuum conditions. The acquired outgassing properties complied with the ASTM E 595 standard. Results indicate excellent space environmental tolerance in the prepared coatings. Thus, the present study established a facile roll-to-roll wet coating process for developing SQ coating on two types of polyimide base films.