Fibrous composites are finding more and more applications in aerospace, automotive, and naval industries. They have high stiffness and strength to weight ratio and good rating in regards to life time fatigue. Investigating mechanical behavior under dynamic loads to replace this material is very important. In the present article, investigate Fatigue of Unidirectional Fibrous Composites by using finite element analysis. So, to achieve this purpose Firstly, modeling fiber and matrix in separate case and simulated semi actual conditions, attained S-N curve of fiber and matrix and after that by using micromechanical model of combination fiber and matrix can approach S-N curve of Unidirectional Fibrous Composites. Finally, Comparisons of the finite element analysis of Ansys and the experimental predictions indicate based on three point bending fatigue testing that the results are satisfactorily in good agreement with each other which approves the power law assumption in the model.