In this paper, we investigate the impact of delegated monitoring by the group leader on group loan default rate in the group-based lending program of the Vietnam Bank for Social Policy (VBSP). We interviewed group leaders from 225 groups in the Mekong River Delta, Vietnam. This group-based lending program employs a unique monitoring system by hiring the group leader to supervise the group. We use two independent variables-commission on business income rate and group size-to measure the delegated monitoring motivation of the group leader. Additional variables serve as proxies for screening, monitoring, and enforcing loan repayment. The results of the Tobit model demonstrate that commission received by the group leader is effectively in reducing group loan default rate within a benchmark. Beyond the benchmark, the likelihood of group loan default increases. Additionally, under the delegated monitoring scheme, group size plays an important role in decreasing group loan default rate. And geographic proximity also helps to make the monitoring by the group leader more effective.