Backgrounds : Tonsillectomy is a commonly performed procedure, many factors could cause post-tonsillectomy haemorrhage and if it occurred patients might need stop bleeding under general anesthesia. Objective : To identify the factors that could cause post-tonsillectomy hemorrhage. Study design : Retrospective cohort and descriptive study Materials and methods : Data have been randomly collected from 549 patients through the records at Phramongkutklao Hospital (PMK) during 2011-2020. The illustration of the data before the procedure, indications for evaluation before the operation, during operation, and post-tonsillectomy have been reviewed to evaluate the risk factors causing the hemorrhage. Results : Our study revealed that the average age was 26.89 ± 15.69 years and overall post-tonsillectomy hemorrhage was 56 (10.2%) patients that occurred often in post-operative day 6. The multiple logistic regression indicated the factors that caused such haemorrhages. There were male gender 13.27% (P=0.039, OR 2.11, CI 1.04 - 4.30), age below 19 years 7.14% (P=0.016, OR 7.96, CI 1.46 – 43.27), age between 19-45 years 13.44% (P=0.024, OR 5.74, CI 1.26 – 26.11), surgeons experience that less than 15 years 14% (P=0.023, OR 2.85, CI 1.16 – 7.01), mean arterial pressure(MAP) 90.00 ± 13.15 mmHg(P=0.02, OR 1.04, CI 1.02-1.07), prescribed intravenous antibiotics 15% (P=0.036, OR 2.03, CI 1.05-3.92) and post-operative pain score in first date over 5 scores 18.67% (P=0.037, OR 2.25, CI 1.05 - 4.81) Conclusion : Patients admitted for tonsillectomy with male gender, age below 45 years, and patients who underwent tonsillectomy by surgeons experience lower than 15 years should be carefully monitored for post-tonsillectomy hemorrhage.