This study explores the experience of social workers who support the decision-making of the most severe developmental disabilities in residential facilities using qualitative case research methods. According to the analysis, more than anything else, the most severe developmental disabilities had difficulty even in various personal preferences and basic interactions. Accordingly, in decision-making situations, social workers rely on individual abilities and judgments. Into the bargain, social workers didn’t have enough educational opportunities to support the most severe developmental disabilities, so that they were building up practical knowledge and gained experience in the field of practice. Third, it was found that sufficient qualitative communication and relationship formation are critical in the decision support process for the most severe developmental disabilities in daily life. Not only that, the facility environment and cultural characteristics were also found to affect the direction of decision support. Finally, it was found that efforts to explore their desires and preferences should be doubled if communication is very limited or a challenging behavior is very serious among the most severely developmentally disabled. Based on the results of these studies, we presented related implications for supporting social workers' decision-making for the most severe developmental disabilities.