An object is to provide a readily fusible glass fiber composition that can alleviate environmental problem and reduce raw material cost by decreasing boron content, and that can facilitate the manufacturing of fine-count glass filament. A glass fiber composition of the present invention is an oxide glass composition, and has compositions of 0.01 to 3% of P2O5, 52 to 62% of SiO2, 10 to 16% of Al2O3, 0 to 8% of B2O3, 0 to 5% of MgO, 16 to 30% of CaO, and 0 to 2% of R2O(R═Li+N+K), which are in terms of oxide represented in mass percentage.