Abstract: A model is proposed for predicting the effective longitudinal shear modulus of fiber reinforced composite materials. The cross section aspect ratio of fiber and the distribution of matrix surrounding are taken into consideration in the model. It is applicable to a full range of aspect ratios. Numerical examples are provided to verify the model. It is found that the longitudinal shear modulus is sensitive to aspect ratios. A comparison is made between the numerical and experimental result to demonstrate the reliability of the present model.