Balancing instant gratification versus delayed, but better gratification is important for optimizing survival and reproductive success. Although psychologists and neuroscientists have long attempted to study delayed gratification through human psychological and brain activity monitoring, and animal research, little is known about its neural basis. We successfully trained mice to perform a waiting-and-water-reward delayed gratification task and used these animals in physiological recording and optical manipulation of neuronal activity during the task to explore its neural basis. Our results showed that the activity of DA neurons in ventral tegmental area (VTA) increases steadily during the waiting period. Optical activation vs. silencing of these neurons, respectively, extends or reduces the duration of waiting. To interpret this data, we developed a reinforcement learning (RL) model that reproduces our experimental observations. In this model, steady increases in DAergic activity signal the value of waiting and support the hypothesis that delayed gratification involves real-time deliberation.