Abstract: A micro beam model was developed to predict the deformation and micro stress distribution of fiber yarns and matrix in three dimensionally woven composites. The shear deformation in the interlayer matrix between adjunct fiber yarns caused by yarn bending was taken into account. In this model,the fiber yarn is modeled by beams in series, therefore its tension and bending deformations are coupled. To a typical three dimensionally woven composite material the method of determining microstructure by woven parameters is discussed. An analytical cell is adopted which is constructed by a least periodic section of yarn and interlayer matrix. Micro stresses of the cell were studied by this beam model. The macro modulus of this material was obtained by averaging micro stresses of the cell. The predictions agree well with those from material test and micro brick finite element method. Analysis reveals that the micro stress caused by the tensile/bending coupling effect is not ignorable.