Two experiments were conducted to explore whether young children could understand the notion of promises. Five and six year old children were asked to estimate the degree of anger when a promise had been broken. Three types of promise -breaking were considered: promise breaking intentionally, forgetting, and to be an accident. The experiment 1 revealed that 6 year-olds could distinguish the accidental promise breaking from the others, and could judge it to be most forgivable. In the experiment 2, stories were prepared where the three types of promise-breaking were combined in one situation while subjects were asked to rank them, in order to have the subjects to show a greater sensitivity to differences between the three types of promise -breaking. The results showed that 6 year-olds could appropriately distinguish the three types of promise-breaking, and that even 5 year-olds were able to judge the accidental promise-breaking to be the most forgivable. These results suggested that even preschoolers have the understanding of others' mental states as well as promises.