The purpose of this study was to explore how motivational factors affect Korean EFL secondary students’ motivated learning behavior using structural equation modeling (SEM). It further examined differences in the internal structure of L2 motivation over time. A total of 489 secondary school students participated in the study. Data were collected at two time points during an academic semester: in the beginning and at the end of the semester. Results of the SEM showed that students’ ideal L2 self was the strongest determinant of their motivated learning behavior at both time points for middle school students, whereas for high school students, ought-to L2 self had the strongest direct and significant impact on their motivated behavior in learning English throughout the semester. It was also found that both middle and high school students’ perceptions about the international posture of English and their parents’ involvement in English learning made indirect effects on their motivated behavior. Pedagogical implications and research suggestions are discussed.