The impedance instrument has nowadays been essential to clinical audiology. There are many clinical reports (Thomsen, Anderson, Jerger etc), but a few experimental reports in Guinea pig (Isaac Eliachar et al) and rabbits (Moeller). The vibratory energy introduced into the external ear canal is changed by the mechanical factor of eardrum itself, the motility of ossicles, the resistance of inner ear system and the air cushion of tympanomastoid cavity. The authors studied how the increase of the air cushion effects on the acoustic impedance of middle ear in dogs (10 samples, 20 ears). The results ars as follows ; 1) The type and maximum compliance of tympanometry are not changed by the increase of air cushion. 2) The static compliance is increased by 0.05cc per 1cc increase of air volume.