PURPOSE: To improve the dielectric constant of a glassfiber composition by blending specified weight ratios of PbO, SiO2 and Al2O3 as essential components and subjecting this blend to melt spinning.

CONSTITUTION: As essential components, by weight, 55 to 70% PbO, 17 to 43% SiO2 and 1 to 10% Al2O3 are blended to obtain a glassfiber composition. In this compsn., as flux components, total ≤1% of alkali metal oxides such as Li2O, Na2O and K2O and impurities such as CaO, MgO, BaO, TiO2, Fe2O3, ZrO2 and SnO2 may be included. This glassfiber composition is heated to melt and spinned, by which glassfiber having high dielectric constant and improved in tensile strength, water resistance and heat resistance can be obtd.