This mixed-method study explored the treatment integrity of virtual reality (VR)-based social skill training for children with high-functioning autism (HFA). In this study, we designed a VR-based social skills training environment using Opensimulator. Data were collected through screen recordings and behavioral observations of HFA children's actions during training. A total of 90 training sessions from 15 children with HFA were analyzed using Jaccard index, a proposed indicator of treatment integrity. To corroborate the Jaccard index scores, we also used thematic analysis to investigate the treatment integrity of the VR-based social skills training. Overall, this study confirmed that VR-based social skill training yielded high treatment integrity. In addition, the results showed that treatment integrity scores differed according to the types of social scenarios in learning activities. Lastly, this study described design implications that should be considered when VR-based social skill training is implemented.