Abstract: Discrepancies of elastic modulus,Poisson ratios and thermal expansion coefficients of fiber reinforced composites were investigated using the constitutive model of Cell and Monte-Carlo method.The results indicate that the discrepancies of fiber reinforced composite effective performance are smaller than the ones of their elements. With the increase of the percentage of fiber volume,the discrepancies of longitudinal effective performance increase, but that of transverse effective performances decrease,except the thermal expansion coefficient in the short-axial direction.The fiber shapes and packing arrays have little effect on the discrepancies of longitudinal effective performance.Composites with hexagonal section fibers and square cross section fibers have the same discrepancies of transverse effective performance.Different fiber packing arrays give different discrepancies.The presented conclusions in this paper are very useful to probabilistic design of composites.