Abstract: This paper shows that only usingthe average strength itself is not enough to express the strength of fibres and composites and that the parameters which represent the distributive state of the strength should also be considered. By calculating their strength on Weibull theory,the quality of a great variety of carbon fibres and C/Al composite wires can be compared.The empiric formula obtained from research shows that when the dispersion degree of the strength is determined by the probability density, the strength can be estimated by the ratio of the curve wide on the half height of the peak to the peak position strength.This paper also shows that the strength of fibres dereases with the increase of the gague length, and their relation is decided by the negative power function in which the absolute value of the index is less than 1, butis not a linear function as was reported in reference(5).