Objectives: To study the association between the mastoid process length (MPL) and surgery success when treating chronic otitis media (COM). Postoperative hearing results and the effect of eustachian tube density and performing mastoidectomy were also explored. Settings: Tertiary referral hospital Participants: We included 127 patients who underwent tympanoplasty with or without intact canal wall mastoidectomy. Main outcome measures: MPL was measured on preoperative computed tomography (CT) scans. The primary outcome of this study is whether MPL is correlated with the success of tympanoplasty at 1 year after surgery. The secondary outcome is the correlation of MPL and the postoperative hearing outcomes. Our analysis used ABG as a binary outcome with cutoff > 10 dB and Gap cposure as a binary outcome with cutoff >= 0 dB. Results: Longer MPL was reversly associated with reperforation of the tympanic membrane at 1 year after surgery in patients who received tympanoplasty without mastoidectomy (odds ratio = 0.89 with 95% confidential interval: 0.79~0.99). The success rate of tympanoplasty with canal wall up mastoidectomy was higher than that of tympanoplasty alone in patients with COM. There was no significant association of MPL with postoperative ABG (odds raio = 1.0) or postoperative gap closure (odds ratio = 1.03). Conclusions: A longer MPL was associated with a favorable surgical outcome when performing tympanoplasty without mastoidectomy. Performing ICWM in addition to tympanoplasty was associated with an improved success rate.