The purpose of this study is to examine the moderated mediating effect of the use of family support services in the relationships among informal social support, family cohesion, and parenting burden of mothers of children with developmental disabilities. For this purpose, 200 mothers who are raising children with intellectual or autistic disabilities were surveyed. The results are as follows. First, the informal social support was associated with parenting burden. Second, family cohesion had a full mediating effect between informal social support and parenting burden. Third, the use of family support services had been shown to moderate the impact of family cohesion on the parenting burden, and the moderated mediating effect were verified. These results suggested that informal social support and practical approach to enhance family cohesion to reduce parenting burden of mothers of children with developmental disabilities, and the need for social policy support and improvement of programs to enhance the family support services.