【Objective】To evaluate the efficacy of interval appendectomy(IA)for simple appendicitis(SA)and complicated appendicitis(CA), focusing on the pathological diagnosis, especially presence of residual appendicitis.【Methods】This was a retrospective cohort study based on data registered in our database from April 2013 to March 2020, of patients who had undergone IA.【Results】A total of 69 patients with SA and 26 patients with CA were included in our study, and the rate of residual appendicitis was higher in the CA group. In the SA group, the length of hospital stay and preoperative interval were not associated with the presence of residual appendicitis. Cases with residual appendicitis had a longer hospital stay and higher rate of postoperative drainage. In 3 of the 4 cases with residual gangrenous appendicitis, persistent residual abscesses were seen on preoperative imaging. Regarding a longer pre-operative interval(120 days and more), the rate of postoperative drainage was significantly lower(0.0% vs. 50.0%, P=0.004)and the operation time was shorter(83 min vs. 113 min, P=0.062).【Conclusions】Our study showed that an interval of 120 days or more to operation and preoperative confirmation of resolution of abscesses can improve the outcomes after IA in cases of CA.