Background: A cancer diagnosis is a very stressful event and has tremendous consequences for most persons who experience it, affecting all aspects of life. Quality of life has become an important patient-reported measurement in chronic diseases especially in cancer assessing how patients feel, physically and emotionally while fighting the diseases could help in improving better care. Chemotherapy treatment despite having therapeutic effects it has many adverse effects, which can affect the quality of life of cancer patients. In Ethiopia factors affecting the Quality of life of cancer patient undertaking chemotherapy treatment is not well explored. Objective: To assess Quality of Life, and associated factors among adult cancer patients undergoing chemotherapy treatment at Amhara National Regional State Referral Hospital oncology treatment centers, Ethiopia, 2021. Methods: An institutional based cross-sectional study was conducted in Amhara National Regional state oncology treatment center from February 15 to April 30 2021. A total of 314 patients were included using simple random sampling. Data were entered into Epi Data 4.6 and analyzed with SPSS version 23. Data were mainly analyzed using descriptive statistics, and factors associated with quality of life were identified by the multivariable binary logistic regression model. A P-value of < 0.05 was used as the criterion for statistical significance and OR with 95% confidence interval was used to indicate the strength of association. Result: The mean age of the study participants was 45.79 years. The mean Quality of Life of cancer patients was 44.32. Multivariable logistic regression revealed that college and above education AOR 4.3 (1.49-12.32), underweight AOR 0.45 (0.24-0.84), >5th cycle of chemotherapy AOR 4 (1.78-9.11), stage IV cancer AOR 0.21(0.06-0.71), comorbidity AOR 0.28 (0.14-0.57), anxiety AOR 0.32 (0.12-0.84), and depression AOR 0.29 (0.13-0.63) were significantly associated with quality of life. Conclusion and recommendation: Quality of life of adult cancer patients under chemotherapy treatment at Amhara National Regional State was poor. Education, anxiety, depression, comorbidity, body mass index, stage of cancer, cycle of chemotherapy were significantly associated with Quality of Life. Therefore, Quality of life assessments, appropriate symptom management, nutritional support, and integration of psycho-oncology care shall be included to improve their quality of life. Keywords: cancer, Quality of life, chemotherapy