Abstract: In this paper, TG/DSC and high temperature XRD were engaged in studying phase transformation behavior of quartz fibers, high resolution SEM was engaged in studying surface micromorphology of heat t reated quartz fibers at different temperatures, and the tensile strength of these quartz fibers was measured by a tensile strength tester. The results indicate that thermal damage of quartz fibers under 1000℃ has two stages: (1) with heat treatment under 600℃, the tensile strength decreases slowly with increasing temperature because diameters of quartz fibers decrease and the original cracks, strip and round bulges on the surface are exposed after surface finishing agent volatilized; (2) with heat treatment at 600～1000℃, thermal stress caused those bulges to flake off and formed some new rifts and cracks. The higher heat treatment temperature is, the more rifts and cracks occur. This is one of the important factors that decreases tensile strength markedly. The results on quartz fiber by TG/DSC and XRD show that though no obvious crystallization occurs, the crystal structure ordering increases after heat treatment under 1000℃ and a little α-cristobalite occurs on the surface causing new bulge defects. This is also another one of the important factors resulting in thermal damage of quartz fibers.