An optical waveguide comprising a substrate, a lower clad layer on the substrate, a core layer and an upper clad layer, at least one of the lower clad layer, the core layer and the upper clad layer is formed of a cured product of a photo-curable organopolysiloxane composition comprising(A) a (meth)acryloyloxy group-containing organopolysiloxane of the following average compositional formula (1): (CH.sub.2.dbd.CR.sup.1COO(CH.sub.2).sub.n).sub.a(Ph).sub.bR.sup.2.sub.c(R- .sup.3O).sub.dSiO.sub.(4-a-b-c-d)/2 (1) wherein R.sup.1 is hydrogen or methyl, R.sup.2 is an C.sub.1 C.sub.8 alkyl or C.sub.2 C.sub.8 alkenyl group, Ph is phenyl, R.sup.3 is hydrogen or an unsubstituted or alkoxy-substituted C.sub.1 C.sub.4 alkyl group, subscripts a, b, c and d are numbers satisfying: 0.05.ltoreq.a.ltoreq.0.9, 0.1.ltoreq.b.ltoreq.0.9, 0.ltoreq.c.ltoreq.0.2, 0