Based on the consideration of that the diaphragm movement during breathing is different in the diaphragmatic ventilation between healthy people and patients with respiratory diseases, this study was aimed to reveal the relationship between diaphragm morphology and respiratory function for the purpose of computer aided diagnosis and treatment. First, a geometric model was generated from CT images of the exhalation and the inhalation states. The ratios of the diaphragm area at the end of expiratory to the diaphragm area at the end of inspiratory were obtained and compared with percentage vital capacities. It is found that there is a positive correlation between the ratio and percentage vital capacity. Furthermore, breathing simulation of the diaphragm was performed by introducing muscle material model representing the direction of muscle fiber. The contraction of diaphragm was reproduced by applying the activity to the muscle material model until the resulted deformation to be consistent with the shape of diaphragm at the end of inspiration. To investigate the effect of deformation in different location of diaphragm, the diaphragm was divided into 11 parts. The contraction ratios, defined as the area at the end of inspiration to the area at the end of expiration, of COPD and ACO patients were compared with those of healthy person. The results show the correlation between the contraction ratio and diseases.