A glass composition for flat cross-section glass fiber includes 50.00 to 60.00% by mass of SiO.sub.2, 9.00 to 18.00% by mass of Al.sub.2O.sub.3, 3.00 to 9.00% by mass of B.sub.2O.sub.3, 15.00 to 30.00% by mass of CaO, 0.10 to 5.00% by mass of MgO, 0.01 to 1.00% by mass of Fe.sub.2O.sub.3, 0.01 to 1.00% by mass of TiO.sub.2, and 0.10 to 2.00% by mass in total of Na.sub.2O and K.sub.2O, in which K.sub.2O/Na.sub.2O is 0.05 or more and less than 0.50, MgO/Fe.sub.2O.sub.3 is 4.10 or more and less than 10.00, and the contents C, M, and T, of CaO, MgO, and TiO.sub.2, and the total content NK of Na.sub.2O and K.sub.2O satisfy the following formula (1). 3.00(CMT/NK).sup.1/23.75 (1)