A glass fiber forming composition exhibits a remarkably high dielectric constant .epsilon..sub.r as well as superior chemical resistance, yet it is readily spun into glass fibers. The composition is characterized to show a devitrification temperature which is lower than a spinning temperature at which the glass composition exhibits a viscosity of 10.sup.2.5 poise, so as to be readily spun into corresponding glass fibers. The composition consists essentially of 40 to 65 mol % of SiO.sub.2 ; 20 to 45 mol % of at least one component selected from the group consisting of MgO, CaO, SrO and BaO; 5 to 25 mol % of at least one component selected from the group consisting of TiO.sub.2 and ZrO.sub.2 ; and 0.5 to 15 mol % of NbO.sub.5/2 as calculated from an incorporated amount of Nb.sub.2 O.sub.5. Alternately, the composition consist essentially of 40 to 65 mol % of SiO.sub.2 ; 20 to 45 mol % of at least one component selected from the group consisting of CaO, SrO and BaO; 5 to 25 mol % of at least one component selected from the group consisting of TiO.sub.2 and ZrO.sub.2 ; 0.5 to 15 mol % of NbO.sub.5/2 as calculated from an incorporated amount of Nb.sub.2 O.sub.5 ; and 0.5 to 15 mol % of AlO.sub.3/2 as calculated from an incorporated amount of Al.sub.2 O.sub.3. The composition is also characterized to incorporate at least 85 mol % of a total amount of the oxides and have a dielectric constant [.epsilon..sub.r ] of 9 or more at 1 MHz and 25.degree. C.