Abstract-Xerostomia is one of the most common complications caused by head and neck cancer radiotherapy. Radiation caused the structure of the salivary glands change thus the saliva production was reduced. This can significantly affect the quality of life of the patients by changing physical, psychological and social function, pain which had an impact on the successful of the radiotherapy treatment. The purpose of this study was to analyze the effect of xerostomia on the quality of life among head and neck cancer patients undergoing radiotherapy at RSUP H. Adam Malik Medan. This was an observational analytic research with cross sectional study approach. 40 subjects of head and neck cancer patients undergoing radiotherapy selected by purposive sampling and data were collected by medical records, anamnesis, and interview. The results of study showed from the 40 subjects, 26 patients (65%) with xerostomia have low quality of life (57.7%), while 14 patients (35%) that not experiencing xerostomia has higher quality of life (85.7%). In conclusion, it is proven that there were significant relationship (p=0.008) between xerostomia and the quality of life among the head and neck cancer patients undergoing radiotherapy.