This study aimed to investigate the factors accounting for science teaching self-efficacy and to examine the relationships among Taiwanese teachers' science teaching self-efficacy, teaching and learning conceptions, technological--pedagogical content knowledge for the Internet (TPACK-I), and attitudes toward Internet-based instruction (Attitudes) using a mediational model approach. A total of 233 science teachers from 41 elementary schools in Taiwan were invited to take part in the study. After ensuring the validity and reliability of each questionnaire, the results indicated that each measure had satisfactory validity and reliability. Furthermore, through mediational models, the results revealed that TPACK-I and Attitudes mediated the relationship between teaching and learning conceptions and science teaching self-efficacy, suggesting that (1) knowledge of and attitudes toward Internet-based instruction (KATII) mediated the positive relationship between constructivist conceptions of teaching and learning and outcome expectancy, and that (2) KATII mediated the negative correlations between traditional conceptions of teaching and learning and teaching efficacy.