Self-determination theory (SDT) explains human motivation by focusing on the importance of motivational regulation based on three basic needs: the needs for competence, autonomy, and relatedness. SDT, when applied in education, emphasizes helping learners internalize extrinsic motivation so as to regulate their learning behavior from an amotivation state to intrinsic motivation. Guided by self-determination theory, the dissertation study was designed for two major purposes: (a) examining the inter-relationships of the components in the self-regulation model to verify its tenability in motivating middle school learners in physical education, and (b) identifying the contribution of the self-regulated motivations to knowledge and skill learning in physical education. Two separate studies were conducted to answer the research questions. In Study 1, 297 sixth grade students from 15 randomly selected middle schools provided need satisfaction and self-regulated motivation data for a two-step structural equation modeling analysis. The results indicated that students' satisfaction of autonomy and competence accounted for a large portion of variability in intrinsic motivation and in identified regulation. Satisfaction of autonomy also contributed to introjected regulation. Satisfaction of any of the needs did not contribute to the external regulation. It was also found that individuals who exhibited satisfaction in competence need lessened amotivation. Unexpectedly, it was found that satisfying the need for relatedness is likely to lead students to becoming amotivated in physical education. In Study 2, 242 participants provided data on SDT components and their learning on health related fitness knowledge and two motor skills determined using a pre- and post-assessment research design. Descriptive statistics showed that students were motivated but learned little. Subsequent structural equation modeling analyses revealed that extrinsic motivation and intrinsic motivation did not contribute to knowledge and skill achievement and amotivation impeded knowledge learning. The findings imply that when competence-based learning achievement is absent, learners can be motivated but do not achieve what they are expected to achieve. The findings provide theoretical insights to developing a constructivist learning environment to direct students' motivation toward learning in physical education and strongly suggest that a curriculum reform in physical education is needed to strengthen competence-based learning (knowledge and skill growth).