Predicting respiratory motion is challenging, due to the complexity and irregularity of the underlying motion pattern. In order to predict respiratory motion, we have developed a biomechanical modeling of the respiratory system which utilizes the 4D scan data (geometrical and mechanical properties) to accurately predict the tumor position due to the deformations and displacement caused by the respiratory movement, to increase the precision of the treatment. The diaphragm is the principal muscle used in the process of respiration. In this case, we introduce a method that enables the simulation of the contractile force generated by the diaphragm muscles. Physiologically, respiratory motion modeling involves the use pressurevolume relationship to apply pressure loading on the surface of the diaphragm. Additionally, the real diaphragm boundary conditions are included to the model and simulated responses are compared to clinical data. Finally, these comparisons show the effectiveness of the proposed physically-based model.