A plastic optical fiber comprising a core polymer having a refractive index of n.sub.1, which comprises a polymer of .alpha., .beta.-unsaturated carboxylic acid fluoroalkylester represented by the general formula [I] as the main monomer, and a clad copolymer having a refractive index of n.sub.2, which comprises perfluoro(2,2-dimethyl-1,3-dioxole) as the main monomer, and which satisfies the relationship of (n.sub.1 -n.sub.2 .gtoreq.0.01): ##STR1## wherein X represents CH.sub.3, H, D, F, Cl, or CF.sub.3, Y represents H or D, and Rf represents a fluoroalkyl group having a linear or branched chain. The optical fiber is prepared by a melt-extrusion method. The optical fiber has excellent light transmitting properties, and it is suitable as an optical fiber for long distance light communication.