An estimation method for mechanical properties of fiber reinforced plastic considering an uncertain microstructure has been described in the paper. As fabric composites are basically made of fiber bundles which can be treated as unidirectional FRP, the mechanical characterization of unidirectional FRP is very important to characterize fabric composites. Since the filaments in the fiber bundles are arrayed randomly, we have generated several FEM models of unidirectional FRP considering the uncertain arrangement of filaments, and estimated the mechanical properties such as stiffness and strength. As the numerical result, the mechanical properties, especially transverse strength has dispersed largely. In addition, the strength decreases caused by residual thermal stresses.