With the advent of positive psychology, clinical attention has shifted from maladaptive/dysfunctional aspects of human psychology to people’s adaptive/functional aspects. Considering this paradigm shift, this study constructed story-sharing activities expressing themes of gratefulness that were suitable for preschoolers and examined effects of the activities on preschoolers’ grateful dispositions (Gratefulness), empathetic abilities (Empathy), and feelings of happiness (Happiness). For this purpose, the study recruited 5 year-old preschoolers from two kindergartens in Seoul: 23 preschoolers from the one kindergarten were placed in the experimental group and 24 preschoolers from the second kindergarten were placed in the control group. The experimental group was given 22-session grateful story-sharing activities for 11 weeks (twice a week, 20-25 minutes per session) and the control group was given the corresponding 22-session free play activities. To examine the efficacy of the treatment, levels of Gratefulness (Gratefulness awareness, Gratefulness expression, and Gratefulness total), Empathy (teacher-rated Empathy and preschooler-rated Empathy), and Happiness were measured at pre-, post-, and 1-month follow-up (FU) time points. FU measurements were made only for the experimental group. Two-way ANOVAs with group (experimental and control) and measurement point (pre and post) as independent variables revealed significant interaction effects for all six preschooler outcomes. Further analyses testing pre-post score differences in each group showed significant pre-post improvement in all six preschooler outcomes in the experimental group but not in the control group. Moreover, treatment gains were maintained at the 1-month FU in the experimental group. These findings suggest that grateful story-sharing activities can be of significant benefit to preschoolers’ grateful dispositions, abilities to empathize, and feelings of happiness.