To fabricate the low-oxygen SiC fibers with excellent thermal stability, electron-beam irradiation polycarbosilane fibers were fired for holding time of 3.6, 18 and 36 ks at each temperature of 1273 to 1573 K, and subsequently the high-temperature exposure test was carried out at 1873 K. As-fired fibers and exposed fibers were characterized by thermogravimetry, chemical analysis, Auger electron spectroscopy, resistivity measurement and tensile test. The strength of both as-fired fibers and fibers exposed to 1873 K increased with prolonging the holding time of firing treatment below 1473 K. On the contrary, a long time firing treatment at 1573 K was no longer effective for the strengthening of the low-oxygen SiC fiber.