Objective: To investigate the risk factors of relapse after allogeneic hematopoietic stem cell transplantation (allo-HSCT) in patients with t (8;21) acute myeloid leukemia (AML) . Methods: The clinical features of patients with t (8;21) AML who received allo-HSCT between January 2008 and October 2020 in the Hospital of Blood Disease and the Chinese Academy of Medical Sciences were retrospectively analyzed. Univariate and multivariate analyses were performed on the factors that might influence relapse. Results: A total of 73 patients were enrolled. The analysis revealed that, out of the 73 cases, 10 had relapses, with a 3-year cumulative incidence of relapse (CIR) of 15.7% (95% CI 7.3%-26.8%) . The median time of relapse was 9.2 (2.0-47.6) months. Furthermore, 19 cases died, with a 3-year overall survival (OS) of 68.9% (95% CI 56.4%-81.4%) . Compared with the RUNX1-RUNX1T1 at first diagnosis, a ≥ 3-log reduction within 3 months and/or 4-log reduction within 3-12 months can significantly decrease 3-year CIR after HSCT (13.3% vs 57.1%; 5.1% vs 25.0%, both P<0.001) . Cox multivariate analysis showed that high levels of RUNX1-RUNX1T1 (≥1.58%) on the day of transplantation (day 0) [P=0.006; HR=28.849 (95% CI 2.68-310.524) ] and the flow cytometric analysis of blasts ratio in bone marrow ≥60% at first diagnosis [P=0.015; HR=6.64 (95% CI 1.448-30.457) ] were independent risk factors for relapse. Furthermore, no significant difference in the effect of c-Kit and Flt3 gene mutations on relapse after transplantation was observed (P=0.877 and P=0.773, resp) . The flow cytometric analysis of blasts ratio in bone marrow ≥60% at first diagnosis [P<0.001; HR=8.925 (95% CI 2.702-29.476) ] and the number of courses to achieve complete remission ≥ 2[P=0.013; HR=4.495 (95% CI 1.379-14.649) ] were independent risk factors for OS. Conclusion: Both high levels of RUNX1-RUNX1T1 (≥1.58%) on the day of transplantation (day 0) and the ratio of flow cytometric analysis of blasts in bone marrow at first diagnosis increase the chance of t (8;21) AML relapse after allo-HSCT. Detection of the transcription levels of RUNX1-RUNX1T1 after allo-HSCT at different times could help predict the hazard of relapse.