The purpose of this study is to develop a self-determination assessment scale for people with developmental disabilities that can be used by practitioners in the disabled welfare center. For this purpose, the components and preliminary questions were set up by compiling the results of a total of four focus group interviews conducted twice each for six parents with developmental disabilities and six experts in the field of welfare for the disabled with more than 10 years of practical experience, and prior research reviews. Subsequently, the validity of the preliminary questions was tested by 16 people, including 6 parents with developmental disabilities, 6 field experts, and 4 researchers in the field of welfare for the disabled. Finally, a survey was conducted on 900 social workers who are providing services to people with developmental disabilities at welfare centers for the disabled. As a result of the analysis, the final self-determination assessment scale for people with developmental disabilities was developed consisting of 18 questions with 4 sub-dimensions: contextual self-regulation, self-efficacy, self-understanding, and recognition of rights. This article provides academic implications that it made the self-determination of people with developmental disabilities, one of the key concepts of social models, to a measurable level, and practical implications that more accurate understanding of self-determination level and aspect of people with developmental disabilities can be made.