Aims and objectives: Type 1 Diabetes Mellitus (T1DM) is characterized by the presence of auto-antibodies, self-reactive T-cells. Measurement of islet autoantibodies can assist in the diagnosis of the disease, and the detection of auto-antibodies in non-diabetic individuals indicate that they are at high risk of subsequent development of the disease. Materials and methods: This study is a cross sectional study which aims to determine the frequency of anti-glutamic acid decarboxylase autoantibodies (GADA) and anti-insulin autoantibodies (IAA) in patients with T1DM and their siblings in AL-Thawrah Hospital, Taiz city, Yemen. It was conducted from June to December 2005. Blood samples were collected from 44 T1DM patients, 52 siblings and 16 controls. Sera were obtained from collected blood samples and were analyzed for GADA and IAA using the immunoradiometric assay. Results: GADA was present in 16 out of 44 (36.3 %) patients,3 out of 52 (5.7 %) siblings and 0/16 (0.0 %) of the controls. While, IAA was positive in 28 out of 44 (63.6 %) patients, 6 out of 52 (11.5 %) siblings and 0/16 (0.0 %) controls. Statistical analysis: Differences between groups were assessed using Chi-square test. Statistical tests were performed using SPSS version 20 and P-value less than 0.05 was considered significant. Significantly high positivity of GADA in patients versus siblings and controls was noted (P<0.001). Conclusion: High positivity of IAA in patients versus siblings and controls was noted as well (P<0.01 and P<0.001, respectively).