Abstract: The unified macro and micro mechanics constitutive model was used to investigate the influence of micro-structural geometry,such as fiber shape and fiber packing array,on the composite initial yielding surfaces under multi-axial loading.Under longitudinal stress,composites reinforced by fibers with square cross section have the same yielding stress as the ones reinforced by fibers with cyclic cross section.Fiber packing array has no effect on the yielding stress.In the presence of transverse or shear stresses, the composite initial yielding surface with square cross section fibers is inside of the one of composite with square cross section fibers.Composite with hexagon fiber array has the lowest yielding stress;however,composite with square-diagonal fiber array has the highest.