The objectives of this study are, firstly, to examining the quality of life (QOL) of the patients with Diabetes Mellitus at Nakornchaisri Hospital and, secondly, to investigate the affecting factors thereof. As the information collecting tools, a general interview form and the World Health Organization's Quality or Life Basic Reviewing and Examining Form (WHOQOL BREF) were in this study. 128 diabetic patients with a continuous record of at least one year follow-up case and with no record of any other current diseases or disability prior to being diagnosed as a diabetic patient were interviewed between September and December 2002. The statistical analysis tools were percentage (%), means (X), standard deviation (S.D.) and T-test as well as ANOVA for defining the relation between variations. It was found that means of QOL of the patients with Diabetes Mellitus of Nakornchaisri Hospital was 70.17% provisionally regarded as good. In addition, the factors affecting QOL were education (p = 0.001) occupation (p = 0.004) personal income (p = 0.003) family income (p = 0.000) adequate income (p = 0.000) status in the family (p = 0.035) medical welfare (p = 0.024) hospital admittance due to Diabetes Mellitus or its complications (p = 0.010) symptoms of Diabetes Mellitus (p = 0.000) disability caused by Diabetes Mellitus (p = 0.028) number of complications (p = 0.005) whereas sexuality, marital status, living environment, age at onset of the disease, duration of the disease, presence of foot sores, blood sugar or the range of treatments used were not. The outcome of this study can be used as a guideline for establishing a plan for the health care of patients with Diabetes Mellitus as well as improving out patient services at Nakornchaisri Hospital.