Background and Objectives@#The purpose of the mastoidectomy and tympanoplasty is to improve the hearing by removing the middle ear lesion and reconstructing the hearing mechanism. The purpose of this study is to verify whether several factors, including the presence of mastoidectomy can affect the hearing outcome of tympanoplasty patients.Subjects and Method From 1989 to 2018, 1260 patients with chronic otitis media who had been followed up for more than 6 months after type 1 tympanoplasty were selected. The each patient group was divided into 2 groups based on the last audiometry; Group A (post operative air-bone gap [ABG] ≤10 dB HL) and Group B (post operative ABG >10 dB HL). We analyzed the various factors including patency of E-tube or mastoid aeration for each group to figure out which factors affect the post operative hearing outcome. @*Results@#In conclusion, even with mastoidectomy, the final hearing after surgery was worse than that of the group who underwent tympanoplasty only (final ABG; tympanoplasty only vs. tympanoplasty with mastoidectomy; 6.59±9.09 vs. 9.71±8.94). In both with and without mastoidectomy group, the size of the perforation and external ear canal diameter before surgery did not affect the final hearing outcome. On the otherhand, in the group with mastoidectomy, mastoid pneumatization and E-tube patency affected the postoperative prognosis, but not in the case without mastoidectomy. @*Conclusion@#In this study, the factors affecting postoperative hearing success were analyzed. The results can be used to predict the postoperative hearing prognosis and to help select surgical treatment.