Reinforced concrete deep (RCD) beams may exist in many structural applications such as offshore structures, transfer girders, pile caps, tall buildings, and water tanks. The depth of deep beams is much greater than normal in relation to their span. This paper presents a three-dimensional Finite Element Analysis (3D FEA) on (RCD) beams for investigating the effectiveness of using mixed steel and FRP bars as the main longitudinal reinforcement to enhance the flexural behavior of RCD beams; optimizing the Af/At ratio. The results of numerical simulation compared with the experimental results which were tested by other researchers proved a good agreement. A parametric study is carried out to investigate the influence of FRP to steel reinforcement ratio Af/At, and concrete strength in the behavior of RCD beams. The results indicated that the flexural load and maximum deflection of RCD beams with high compressive strength concrete improved, and the load capacity increased as the Af/At ratio increased.