A UV-curable and foamable resin composition comprising a photopolymerizable urethane acrylate oligomer, a photopolymerable monomer, a photo polymerization initiator, a photolytic foaming agent selected from an azo compound, a combination of sulonium salt and an inorganic carbonate, and a mixture thereof, and a photolysis catalyst is useful for coating a fiber or ribbon bundle to be applied to an air blown fiber system.