Objective:To investigate the drug resistance factors in postoperative gemci-tabine chemotherapy after radical resection of pancreatic cancer.Methods:The retrospective case-control study was constructed. The clinicopathological data of 255 patients with pancreatic cancer who were firstly admitted to the Department of Hepatobiliary Surgery of the First Affiliated Hospital of Xi ′an Jiaotong University from January 2018 to June 2021 were collected. There were 140 males and 115 females, aged (59±10)years. All patients underwent radical resection of pancreatic cancer and received postoperative gemcitabine-based adjuvant chemotherapy. Observation indicators: (1) follow-up; (2) postoperative chemotherapy; (3) drug resistance and changing of regimen; (4) factors influencing postoperative chemotherapy resistance. Measurement data with normal distribution were represented as Mean± SD, and comparison between groups was conducted using the independent sample t test. Measurement data with skewed distribution were represented as M( Q1, Q3), and compari-son between groups was conducted using the Mann-Whitney U test. Count data were described as absolute numbers, and comparison between groups was conducted using the Pearson chi-square test. Univariate analysis was conducted using the corresponding statistical methods based on data type. Multivariate analysis was conducted using the Logistic regression model with forward method. Kaplan-Meier method was used to draw survival curve, and Log-Rank test was used for survival analysis. Results:(1) Follow-up. All 255 patients were followed up for 18.6(16.7,21.4)months. The median survival time of 255 patients was 18.2[95% confidence interval ( CI) as 15.8-20.6]months. (2) Postoperative chemotherapy. Of the 255 patients, there were 5 cases receiving postoperative chemotherapy as gemcitabine monotherapy, 167 cases receiving postoperative chemotherapy as the AG combination (gemcitabine plus albumin-bound paclitaxel), 74 cases receiving postoperative chemotherapy as the GS combination (gemcitabine plus S-1) and 9 cases receiving postoperative chemotherapy as the GP combination (gemcitabine plus platinum). (3) Drug resistance and changing of regimen. Of the 255 patients, 81 cases completed the course of postoperative chemotherapy and evaluation. Of the 81 patients, there were 18 cases with no recurrence or metastasis of tumor, 10 cases with tumor local recurrence, 40 cases with tumor lymph node metastasis or distant metas-tasis, 3 cases with tumor local recurrence combined with distant metastasis, 10 cases with elevation of CA19-9. Of the 81 patients, 18 cases responded to chemotherapy, 63 cases underwent resistant to chemotherapy, including 11 cases with primary resistance and 52 cases with acquired resistance. The 63 patients with chemotherapy resistance underwent changing of regimen. (4) Factors influencing postoperative chemotherapy resistance. Results of multivariate analysis showed that chemotherapy cycle<6 is an independent risk factor for postoperative chemotherapy resistance in patients ( hazard ratio=17.18, 95% CI as 2.07-142.28, P<0.05). Conclusion:Adjuvant chemotherapy cycle <6 is an independent risk factor for postoperative chemotherapy resistance for gemcitabine based chemo-therapy in pancreatic cancer patients receiving radical resection.