In this study, the influence of the duration of opening vestibular window and the application of suction in auditory bulla on cochlear function and ultrastructures were observed by ECochG and scanning and transmission election microscopy in 44 guinea pigs divided into six groups. The results were as follows: (1) The CAP thresholds progressively enhanced as the prolongation of duration in some of outer hair cells in the basal turn occured 180 minutes after total stapedectomy. (2)Significant CAP threshold shift and irreversible pathological changes in some outer hair cells in the basal turn of cochlea were found with continuous suction for 60 minutes in auditory bulla after total stapedectomy. (3)CAP threshold shift without damage of outer hair cells could be fourd with the application of suction for 60 mimutes in auditory bulla before stapedectomy. The results suggested that opening vestibular window for longer time with suction in auditory bulla might be one of the factors to cause postoperative sensorineural hearing loss in total stapedectomy. But the suction noise might not be enough to cause postoperative sensorineural hearing loss in tympanoplasty due to its relatively low intensity.