The properties of four kinds of Type II carbon fibers, which had been precarbonized at 300 °C, 400 °C, 500 °C, and 600 °C, respectively, during two-stage continuous carbonization, were measured after being air oxidized for periods of 1 to 6 min at 550 °C. The effects of precarbonization temperature on mechanical properties, density, morphology, elemental composition, and microstructure of the carbon fibers during the air oxidation are discussed in this article. The precarbonization process strongly affected the surface properties and mechanical properties of the final oxidized carbon fibers. The carbon fibers developed from the different precarbonization temperatures all had different structures. The carbon fibers that had been precarbonized at 300 °C had a more ordered structure than other fibers after air oxidation. These carbon fibers also had a higher performance than the other fibers. Carbon fibers also showed different oxidation behaviors caused by differences in surface morphology resulting from each different precarbonization temperature. Optimum conditions not only improved the tensile strength and modulus, but also increased the density and oxygen content. Experimental results showed that the tensile strength of the carbon fibers precarbonized at 300 °C increased from 2.4 GPa to 4.3 GPa (80%) after 6 min oxidation at 550 °C.