This study includes an analytical investigation of a composite beam consist of concrete, structural tees, and the reinforcement steel. The plastic hinge length theories are applied to this composite beam to calculate maximum deflection exactly. This composite beam can reduce the floor height compared with the reinforced concrete structure with same internal force. To calculate maximum deflection, we used plastic hinge length suggested by Corley, Sawyer and Mattock. Later, the experimental investigation should be performed to find out the plastic hinge length exactly.