Pancreatic islet transplantation is an ideal therapy for type I diabetes. Interestingly, small size of islets may exhibit superior performance in transplantation in terms of function and hypoxia. To this end, large size of islets was dissociated to generate islet single cells and they were re-aggregated to make small size of islet-cell-cluster (ICC). However, the function of insulin secretion from ICCs was damaged due to weak cell-to-cell interaction. To overcome this limitation, here the dispersed islet single cells were re-aggregated at the concave microwell array that was fabricated with poly(dimethylsiloxane) (PDMS) by using soft-lithography and mold-replication technology. The aggregated ICCs had uniform size, and their insulin secretion in response to the change of low and high glucose concentration was similar to that of intact islets since the cell-cell interaction of islet single cells was strongly induced due to the shape of concave microwell. Also, after cultivation for 2 weeks, the expression of hypoxia-induced factor-1${alpha}$(HIF-1${alpha}$; a hypoxia marker protein) in the ICCs was rare; meaning that hypoxic state in the central area of ICCs was attenuated. On the other hand, the ICCs generated in flat-bottomed wells had abnormal insulin secretion caused by weak cell-to-cell interaction. In addition, after cultivation for 2 weeks, the generated ICCs in flat-bottomed well had weak signal of HIF-1${alpha}$, while intact islets had its strong signal. Therefore, new culture system using concave microwell array may be a promising alternative to success fully prepare functional ICCs.