A composition comprising (a) an organopolysiloxane having a unit of the formula: RSiO.sub.3/2 wherein R is a substituted or unsubstituted monovalent C.sub.1-10 hydrocarbon group in its molecule and having at least 0.05 alkenyl groups each directly attached to a silicon atom per silicon atom, (b) an organohydrogenpolysiloxane having at least 2 hydrogen atoms each directly attached to a silicon atom per molecule, and (c) a platinum catalyst, maintains good light transmission in cured form when exposed to humidity or stresses and is thus suitable for forming cores of optical fibers.