Objective : The experiment consists of measuring functional grasp ability of a child suffering from sensory defense disability of the hands as the child is treated with sensory integration therapy. Methods : We treated two-year-old child with developmental disability to set up the reversal design(ABAB design). We used treatment equipments, free activities, and toys for sensory integration therapy. Results : A child's functional grasp ability did not show anything before the sensory integration therapy, however, after the sensory integration was applied during the first treatment(Treatment 1), the five minutes before the treatment, it averaged 1.87 times(ranges=0 to 5 times), the five minutes after the treatment, it averaged 1.25 times(ranges=0 to 8 times) showing an increased. After ending the sensory integration therapy, the functional grasp ability averaged 2.5 times before the therapy and 4 times after the therapy, which showed a totally different tendency, when we reapplied the treatment during the second treatment(Treatment 2), it averaged 2.4 times(ranges=1 to 4 times) before the treatment, and it averaged 5.3 times(ranges=4 to 6 times) after the treatment, which showed a big improvement. Conclusion : The sensory integration therapy brought an increase in functional grasp ability along with improvement in daily life functions of the child. For this result, there should be more research to see more effects and results to help those in need.