The present study examined preschoolers' understanding of the relationships among others' emotions, desires and prosocial behaviors. Adults and children(4-, 5-, 6-year-olds) were asked to indicate the protagonist's feelings(happy, sad, or "just okay") after listening to a series of six stories. The stories varied on (1) whether the protagonist's desire was fulfilled and (2) whether the protagonist shared a desired object with the other story character. Both adults and children reasoned that the protagonist would be happier when her/his desire was fulfilled than when her/his desire was not fulfilled. When the protagonist's desire was fulfilled, both adults and children attributed more positive emotions to the protagonist who shared her desired object with someone else than to the one who did not share her desired object. When the main character's desire was not fulfilled, children, but not adults, fail to reason that sharing behaviors could be related to feeling less of negative emotions. The results demonstrate children's understanding that others' emotions can be affected by others' prosocial behaviors as well as others' desires.