The research aims to improve the social competence of the students of SMK Muhammadiyah 1 Sleman through social skill training. The method used in this research was quantitative with the action research format. The model referred to Kemmis and Taggart model. The research subjects were the tenth-grade students who had low social competence. The data collection methods used in this research were scale observation and interview. The Instruments used were social competence observation guide and

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... view guide. The data analysis technique was quantitative descriptive technique with percentage. The research result showed that social competence could be improved through social skill training consisting of six steps: defining the target skill, modeling, role playing, giving feedback, positive strengthening and homework. This could be seen from each participant's social competence before treatment to social competence level after the first cycle treatment and the second cycle treatment. After the first cycle treatment was conducted, as 31% of the students reached high social competence. And when the second cycle treatment was conducted, 100% of the students reached high social competence and already reached the research target, each student having minimum social competence 75%. Generally, when the students were in the classroom, they dared to express their opinion and gave responses to their classmates. The students were able to manage their shyness, and when did not understand in the lesson, when they were brave to ask.