The purpose of this study is to investigate the relationship among self-efficacy, motivation proposed by self-determination theory and academic achievement and to confirm the relative predictive power of motivation variables in predicting 480 middle/high students' achievement. The instruments used in this study were 'Self-efficacy' and 'Self -regulation' scale. The data were analyzed with Pearson correlations and multiple regression (stepwised) analysis. The correlations among self-efficacy,

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... f-determination motivation and academic achievement were significant. The variables among self-determination were correlated to academic achievement in middle school. Among the variables of self-determination motivation, identified regulation, Integrated regulation and intrinsic regulation were positively related to academic achievement in high school but amotivation was negatively related to academic achievement. Self-control efficacy and self-confidence were the significance predictor of academic achievement in middle school. Hard-task preference and self-control efficacy were the significance predictor of academic achievement in high school students. Introjected-regulation was the best predictor of academic achievement in middle school. Identified-regulation was the best predictor of academic achievement in high school.