This study was conducted to determine the effects of a horticultural therapy (HT) program, based on B.F. Skinner’s behavior modification theory and special education science curriculum for Korean children with intellectual disabilities for the improvement of attention and sociality. Twenty-four participants (10 males, 14 females, in grades 1 to 3) with intellectual disabilities were recruited from a special education class at an elementary school in Seoul, South Korea. Twelve children participated in the HT program after-school for 6 months (Mar. to Aug. 2009, once per week, ≈40 min per session); the control group consisted of the remaining 12 children. Before and after the HT program, Conners’ teacher rating scales—revised and the social skills rating system assessments were conducted by parents/caregivers or teachers for each of the children. Analysis of covariance (ANCOVA) and chi square tests were used to compare differences between the two groups. Difference in attention was not significant between groups. Children in the HT group had statistically significant higher sociality scores than those in the control group (P < 0.001). In conclusion, the HT program improved the sociality of children with intellectual disabilities. To maximize the therapeutic effects of the HT program for attention, the program should be revised and supplemented based on the results in this study. A larger sample size and factoring in the level of disability and year in school of the participants would increase the precision in assessing therapeutic effects.