Abstract: Student engagement is an important aspect of the use of technology in online learning in this paper. It is energized by motivation and explained by three basic needs in Self-determination theory (SDT). Teacher support distinguished in SDT was widely applied in face-to-face settings, but not online learning, particularly in K12 context. We know very little how to support the needs of the young children in online learning. Recently, the founders of SDT also stated that we need more studies to understand how to support students’ needs in online learning environments. Therefore, this study aims to investigate how well three teacher support dimensions distinguished in SDT – autonomy, structure and involvement – encourage K12 student behavioral, cognitive and emotional engagement. In this study, three hundred and thirty Grade Eight students learned online for four weeks, and finished a questionnaire on perceived teacher support and their engagement. Stepwise multiple regression models were used to analyze the data. The two major findings are teacher involvement is the most influential predictor and autonomy support has less effect. Two plausible explanations are (i) teacher-student relationships are more important in online learning due to the school nature and (ii) teachers can support autonomy less in online learning that offers more freedom learning. experience due to its less structure.