Concrete encased steel beams have been used extensively in bridges and buildings construction for many years. Investigations showed that such composite beams lead to a reduction of both structure weight and the resulted deformations. In the present paper, an accurate modeling for concrete encased steel beams is achieved by using the nonlinear three-dimensional finite element method through the general purpose computer program DIANA. The analysis takes into consideration the interaction between the steel and concrete to simulate the behavior of the encased beams well. The analysis has been performed for both linear and nonlinear stages. The results obtained from the present work are compared with the corresponding ones of previous available experimental works. The comparison showed that the present model is capable of introducing a good representation for the concrete encased steel beams behavior and strength.