Abstract: Provided is an optical fiber having a primary coating thereon wherein the coating is formulated from an ultra-violet light curable resin composition containing:(A) a polyether polyol compound containing, as structural units, groups represented by the following formulas (1), (2, and (3),(1) --CH.sub.2 CH.sub.2 O--(2) --CH.sub.2 CH(R)O--, and(3) one or more groups selected from groups (a), (b), and (c) below, ##STR1## wherein R represents an alkyl group containing two or more carbon atoms, and R.sup.1 and R.sup.2 are independently selected from a hydrogen atom or a methyl group;(B) a polyisocyanate compound; and(C) a (meth)acrylate compound having a hydroxy group.