A bench-scale continuous mode microwave plasma assisted carbon fiber producing system, where oxidized 12K PAN precursor was carbonized by microwave plasma, was manufactured and a series of experiments was conducted in order to find the effects of microwave power on the physical property and strength of the carbon fibers. It was found that as the microwave power was increased from 0.73 Wh/g to 5.2 Wh/g, the ultimate tensile strength increased from 0.18 to 1.29 GPa and further increase of the microwave power did not increase the ultimate tensile strength. On the other hand, as the microwave power was increased from 0.73 Wh/g to 5.8 Wh/g, the Young’s modulus continuously increased from 9.8 to 132 GPa. The results of this study indicated that the optimum microwave power was 5.2 Wh/g. Key