The effects of carbonization temperature on the structural defects and mechanical properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscope (AFM) and mechanical properties test. The results show that, in the stage of low temperature, strength and modulus both increase with temperature increasing, but they show different increasing rate lower and higher than that in 700℃. When temperature is lower than 700℃, the stress-strain curves demonstrate there are “zigzag” inflections due to rapid decreasing of stress without obvious yield. Above 700℃, yield appears prior and then inflection. In the stage of high temperature, the fracture of fiber presents typical characteristics of brittle material .The changes of mechanical properties are related to the changes of structure. The increase of strength is due to the crystallinity of quasi-crystal graphite, while the increase of modulus is mainly depended on the improvement and packing order of the quasi-crystal graphite. The fracture of carbonized fibers in 500-1350℃ mainly depends on the surface defects. The slender cracks along the fiber axis are both on the surface of carbonized fibers in 700℃ and 1350℃. The slender crack diameter of carbonized fiber in 700℃ is about 2 nm, and the length is from dozens of nanometers to hundreds of nanometers. The slender crack diameter of carbonized fiber in 1350℃ is about 1 nm, and the length is in the range of 50-190 nm. These slender cracks may be the main reason for the fracture of the fibers.