Because of good mechanical properties of the fiber composites, the fiber bundles without the resin curing process and with the features of high modulus, high strength and good performance of creasing resistance etc, can be used in prestressed structures. According to the strength properties of the fiber, the tensile characteristic function of the fiber bundles is derived. The tensile test is designed and the parameters of the weibull distribution are estimated based on the result of the test. The ultimate strength of fiber is inversely proportional to the length of fiber. The calculating strength should be reduced in design, because the discreteness of the strength of the fiber bundle in test is much larger than the calculated values.