The Social Competence Observation Scale (SCOS) is an objective observational measure developed to assess the social performance of children with autism spectrum disorder (ASD). This paper describes development of the SCOS and results of initial testing of reliability and treatment sensitivity for a sample of 12 children, ages 4-6 years, with ASD. Based on observations, the measure can yield three scores including a social impairment severity, change, and responder status score. Results suggested good-to-excellent reliability (inter-observer agreement and test-retest) and the SCOS was treatment sensitive to changes resulting from the intervention (at the social impairment severity score, change score, and responder status levels). Lastly, the effect size based on the SCOS social impairment severity score improvements (baseline-to-post-treatment) for the coders was large and was comparable to parent ratings on a standardized measure of ASD social impairment symptom severity. Implications and suggestions for future study are discussed.