Type 1 diabetes mellitus (T1D) is an immune-mediated disease. The autoreactive T cells in T1D patients attack and destroy their own pancreatic cells. In order to systematically investigate the potential autoreactive T cell receptors (TCRs), we used a high-throughput immune repertoire sequencing technique to profile the spectrum of TCRs in individual T1D patients and controls. We sequenced the T cell repertoire of nine T1D patients, four type 2 diabetes (T2D) patients, and six nondiabetic controls. The diversity of the T cell repertoire in T1D patients was significantly decreased in comparison with T2D patients (P = 7.0E−08 for CD4+ T cells, P = 1.4E−04 for CD8+ T cells) and nondiabetic controls (P = 2.7E−09 for CD4+ T cells, P = 7.6E−06 for CD8+ T cells). Moreover, T1D patients had significantly more highly-expanded T cell clones than T2D patients (P = 5.2E−06 for CD4+ T cells, P = 1.9E−07 for CD8+ T cells) and nondiabetic controls (P = 1.7E−07 for CD4+ T cells, P = 3.3E−03 for CD8+ T cells). Furthermore, we identified a group of highly-expanded T cell receptor clones that are shared by more than two T1D patients. Although further validation in larger cohorts is needed, our data suggest that T cell receptor diversity measurements may become a valuable tool in investigating diabetes, such as using the diversity as an index to distinguish different types of diabetes.