Abstract: Objective To explore Ki-67 expression in the histological subtypes of lung adenocarcinoma and its prognostic significance. Methods A total of 171 lung adenocarcinoma patients who underwent surgery in the Affiliated Hospital of Xuzhou Medical University from January 2018 and December 2020 were enrolled and their tissues and clinical data were retrospectively analyzed. The levels of Ki-67 in lung adenocarcinoma tissues were detected by immunehistocheistry. The relationship between Ki-67 expression and histological subtypes and other clinicopathological characteristics of lung adenocarcinoma were evaluated. The relationship between Ki-67 expression and the prognosis of lung adenocarcinoma was evaluated. Results The expression of Ki-67 was various in different histological subtypes of lung adenocarcinoma. The expression rate of Ki-67 was 15% (5%-20%) for lepidic predominant adenocarcinoma, 15% (10%-30%) for acinar type, 10% (7.5%-35%) for papillary type, 25% (10%-67.5%) for micropapillary type and 30% (25%-50%) for solid predominant adenocarcinoma (P＜0.001). The expression of Ki-67 in grade 1, grade 2, and grade 3 was 12.5% (5%-20%), 10% (5%-30%), and 30% (20%-40%), respectively (P<0.001). Patients with solid or micropapillary components showed high Ki-67 expression (P<0.001), while those with lepidic components presented low Ki-67 expression (P=0.032). Furthermore, the increased expression of Ki-67 was associated with men, smokers, large tumor size, lymph node metastasis, pleural lymphatic vessel invasion, high TNM stage and solid nodule. Multiple logistic regression analysis showed that histological subtypes, and tumor size were the influencing factors of high Ki-67 expression. KM survival analysis indicated that increased Ki-67 expression was significantly associated with decreased disease-free survival (χ²=7.598, P=0.006). Conclusions The expression of Ki-67 is related to three-tier grading system and grading systems based on predominant histologic subtypes. High expression of Ki-67 in lung adenocarcinoma is associated with poor prognosis.