End to side anastomosis between the trachea and the bronchus was widely employed as one of bronchial reconstructed method. However, functional evaluation undergone bronchial reconstruction did not completely display its superiority functionally. This study was undertaken to determine whether end to side anastomosis was far superior in regard to reserved pulmonary function by means of observation on changes in size of anastomotic area on xp film delineated during inspiration and exspiration simultaneously and also by intrabronchial pressure tracing. The results were as follows. 1) The creation of window defect on the tracheal wall prior to anastomosis was necessary to maintain an adequate anastomotic area. 2) The end to side anastomosis obliquely results in an excellent ventilation functionally. In contrast, the ill effects were disclosed by moderate angulation as far as exceeded 45 degree against tracheal wall owing to loss of rigidity in bronchial wall. From the present study, we concluded that end to side anastomosis obliquely with moderate degree should be avoided because of bronchial stenosis produced by bronchial collapse due to loss of supporting ability of bronchial cartilage