To obtain a coated fiber protected from influences of the external environment by coating a fiber with a photopolymerizable compsn. essentialy comprising epoxidized polydiene oligomers.

The hydrophobic material to cover a fiber is obtd. by photopolymn. of a compd. containing at lest one kind of epoxidized polydiene oligomer and at least one kind of photoinitiator, and if necessary, containing a reactive diluent. As for the fiber, for example, fabric fibers, metal fibers and glass fibers are preferably used. The diene is a conjugate diene and selected from butadiene, isoprene, chloroprene, etc. The epoxidized polydiene oligomer is preferably a hydroxytelechellic polybutadiene diol having OH groups on both ends of the chain. The obtd. fiber has high durability against hydrolysis and excellent mechanical characteristics. The coating film the excellent adhesion property to the fiber and enables cationic photopolymn., and polymn. is not inhibited by oxygen in air.