Polycrystalline near stoichiometric SA type SiC fibers have a prospective application in the fields of the new generation aero engine and hypersonic vehicles due to their excellent temperature resistance. In this work, microstructure evolution, tensile strength as well as fracture behavior of the second-generation domestic F-II SiC and the third-generation SA (F-III) SiC fibers before and after heat treatment were studied. The results showed that F-III fiber was mainly composed of β-SiC grains (~200 nm) and a small amount of free carbon, while F-II fiber was composed of β-SiC grains (~5 nm), free carbon and amorphous SiC_xO_y phase. Compared with the F-II fiber, the F-III fiber showed lower tensile strength at room temperature, owing to their larger grain size and pores. However, after heat treatment at 1800 ℃, the structure and strength of F-III fiber remained almost unchanged, while the strength of F-II fiber decreased sharply due to decomposition of SiC_xO_y phase and grain growth. The excellent high temperature resistance of SA type fiber could be attributed to high crystallinity, large grain size, low carbon and oxygen content in microstructure and composition.