The present invention relates to a UV curable clad resin composition for optical fiber or waveguide comprising: (A) a photopolymerizable acrylate oligomer derived from the mixture composed of a polyol copolymer, an acid anhydride, an acrylate alcohol, a condensation polymerization catalyst and a polymerization inhibitor, (B) a photopolymerizable monomer, (C) a photopolymerization initiator, and (D) a leveling or antifoaming agent. The inventive composition minimizes optical loss and has a controllable low refractive index in the range of 1.35 to 1.46.