Recently, composite beams have been developed in which concrete is filled in a U-shaped steel plate for saving height of story. And due to high flexural stiffness and bending strength, it is widely applied in the field where high load and long span are required. The AU composite beam was improved the instability of the existing beams because it makes a closed section by attaching a cover type shear connection to the existing U - shaped composite beam with open upper section. In this study, AU composite beam resisted by composite-section during the using phase was evaluated the safety through the finite element analysis. The analysis is performed on the five specimens of AU composite beams according to applied deck-system and compared with the results of 2 - point bending test. As a result of the analysis, behavior of beam was shown by integrated composite section. And the evaluation of flexible capacity was p.