Abstract: An organopolysiloxane composition which is adapted for use as a core of plastic optical fibers comprises (A) an organopolysiloxane having a unit of the following general formula (1)RSiO.sub.1.5 (1)wherein R represents a substituted or unsubstituted monovalent hydrocarbon, and also having not less than 0.05 alkenyl groups directly bonded to the silicon atom thereof per unit silicon atom, (B) an organosilicon compound of the following general formula (2) ##STR1## wherein R.sup.1 's independently represent a monovalent hydrocarbon group, Q represents a divalent aromatic hydrocarbon group, and a is an integer of from 1 to 3, and (C) a platinum catalyst for crosslinking between the organopolysiloxane and the organosilicon compound. An optical fiber comprising a core made of a cured product of the composition is also described.