A study is made of the effective axial shear modulus of a fiber reinforced material with random fiber cross-sections so that the micromechanics is governed by stochastic differential equations. A coarse-graining procedure is adopted to investigate the macroscopic behavior of the material. This analysis leads to the formula for the effective axial shear modulus μ ∗ = μ 1 / { 1 − 2 c ( μ 2 − μ 1 ) / ( μ 2 + μ 1 ) } , where μ 1 and μ 2 are the shear modulus of the matrix and fibers respectively and c is the concentration of the fibers less that 0.5 . For 0.5$"> c > 0.5 , the fiber and matrix moduli are to be interchanged and c is to be replaced by 1 − c . The results of this study are compared with those of the theory of fibre reinforced materials. Finally, a numerical example is presented with graphical representation.