Abstract: The theory of two-leveled randomly-enlarging critical core was put forward to predict the tensile strength of unidirectional bundle-reinforced composites. According to the manufacture technologies of FRP, a model was established to calculate the fiber spacing and the quantity of fibers in a bundle. The fibers in composites were treated through 2 levels: fiber bundle, and array of fiber bundles. The father core and son core were put forward so as to precisely describe the structure of a critical core. Beyerleinps formula was adopted to calculate the average stress concent ration factor of the bundle, which is caused by the sequential failure of bundles. Sivasambups formula was applied to calculating the stress concentration factor of the fiber inside bundle, which is caused by the sequential failure of fibers. Based on the propagation mode of fiberps failure, according to the statistics theory, considering the enlarging ineffective length inside the bundle, the formulas calculating the failure possibility of composites were deduced. A computer program was compiled. Through this program, the tensile strengths of plastics reinforced respectively by unidirectional woven rovings of S glass, E glass, and Basalt fiber were predicted. Meanwhile, along with the mechanical performance of the matrix under tension and shearing, the tensile strengths of these FRPs were gained through experiments. Then , the predicted result s were compared with the experiment results. Research result shows that the predicted results, which meet the experiments, can be gained by directly inputting the material and geomet ry parameters of experiment specimens.