162 Background: Cancer is a high risk for worse COVID-19 outcomes. However, the data on factors of severe COVID-19 cancer patients in Thailand are underrepresented. This study aims to evaluate the factors of severe COVID-19 in cancer patients. Methods: From October 2020 - April 2022, cancer patients with SARS-CoV-2 infection confirmed by a positive RT–PCR test or COVID antigen test at Sawanpracharak Hospital were retrospectively analyzed. Severe COVID-19 is defined as requiring a high-flow nasal cannula, or mechanical ventilator. Demographic and risk factors data were collected from the electronic medical record. Univariate and multivariate logistic regression analyses were performed to identify risk factors for severe COVID-19. Results: Among 213 patients, 20 (9.4%) developed severe COVID-19 and the mortality rate was 6.5%. There was a significantly higher rate of discontinued cancer treatment and disease progression in severe than non-severe COVID-19 (73.7% vs 9.9%, P<0.001 and 82.4% vs 25.3%, P=0.001). Univariate analysis revealed that diarrhea (OR 6.042, 95% CI 1.541-23.629, P=0.010), oxygen saturation < 92% (OR 9.625 95% CI 2.968-31.208, P<0.001), ECOG ≥ 2 (OR 4.215 95% CI (1.598-11.114, P=0.004), Best supportive care (OR 4.190, 95% CI 1.328-13.223, P=0.015), diabetic mellitus (OR 5.123, 95% CI 1.866-14.065, P=0.002), hypertension (OR 4.758 (95% CI 1.772-12.775, P=0.002), liver disease (OR 4.362, 95% CI 1.371-13.897, P=0.013), polypharmacy (OR 4.991, 95% CI 1.805-13.799, P=0.002), < 2 vaccines (OR 3.558, 95%CI 1.342-10.188, P=0.018), Neutrophil/Lymphocyte Ratio (NLR) ≥ 12 (8.479, 95% CI 2.833-25.247, P<0.001), CRP≥ 65(OR 3.580, 95% CI 1.111-11.542, P< 0.001), hospital-acquired pneumonia (HAP) (OR 18.920, 95%CI 6.112-58.574, P<0.001) are associated with severe COVID-19. Multivariate analysis found that high NLR (OR 46.787, 95%CI 2.063-1061.227, P=0.012), Liver disease (OR 26.885, 95%CI 1.666-433.785, P=0.020), and hypertension (OR 44.095, 95% CI 1.082-1797.017, P=0.045) were significant factors of severe COVID-19. Conclusions: High baseline NLR, liver disease, and hypertension were independent factors for severe COVID-19 in cancer patients. Therefore, patients with these risk factors should be more carefully monitored.